Rewrite proof decoding code to not use a separate map

[tomaka]

Before this PR, decoding a proof builds a BTreeMap that basically duplicates the trie structure.

Looking things up in the proof then consists in traversing the BTreeMap. It seems to me, however, that we would be better off traversing the proof itself rather than a BTreeMap.

This is what this PR does. Decoding a proof now builds a "proof layout" that is now traversed when looking things up in the proof.
As a side effect, the keys and prefixes are no longer passed as slices by as iterators. This should also slightly improve performances, as there are various locations where we builds Vecs just to pass them as parameter.

Since the objective of this PR is to improve performance, I might throw it away if it turns out to be slower than before (which is why I did #1460 beforehand).

[tomaka]

Some preliminary benchmarking show a 25% to 30% gain both when decoding and when accessing a proof content, and that's not accounting for the fact that we can optimize accessing the proof content further by storing the partial key on the side to avoid decoding trie nodes multiple times.

[tomaka]

Everything now seems to work fine.

[tomaka]

Here are the benchmarks compared to the main branch:

Benchmarking proof-decode/decode/1762: Collecting 100 samples in estimated 5.0073 s (858k iteratio
proof-decode/decode/1762
                        time:   [5.8289 µs 5.8367 µs 5.8439 µs]
                        thrpt:  [287.54 MiB/s 287.90 MiB/s 288.28 MiB/s]
                 change:
                        time:   [-24.676% -24.462% -24.255%] (p = 0.00 < 0.05)
                        thrpt:  [+32.022% +32.384% +32.760%]
                        Performance has improved.
Found 3 outliers among 100 measurements (3.00%)
  1 (1.00%) high mild
  2 (2.00%) high severe
Benchmarking proof-decode/get-storage-value/1762: Collecting 100 samples in estimated 5.0013 s (14
proof-decode/get-storage-value/1762
                        time:   [112.08 ns 112.34 ns 112.61 ns]
                        thrpt:  [14.572 GiB/s 14.607 GiB/s 14.641 GiB/s]
                 change:
                        time:   [-52.757% -52.449% -52.130%] (p = 0.00 < 0.05)
                        thrpt:  [+108.90% +110.30% +111.67%]
                        Performance has improved.
Found 6 outliers among 100 measurements (6.00%)
  2 (2.00%) low mild
  1 (1.00%) high mild
  3 (3.00%) high severe
Benchmarking proof-decode/next-key/1762: Collecting 100 samples in estimated 5.0003 s (14M iterati
proof-decode/next-key/1762
                        time:   [122.81 ns 122.92 ns 123.05 ns]
                        thrpt:  [13.336 GiB/s 13.350 GiB/s 13.362 GiB/s]
                 change:
                        time:   [-40.841% -40.518% -40.201%] (p = 0.00 < 0.05)
                        thrpt:  [+67.227% +68.117% +69.037%]
                        Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
  3 (3.00%) high mild
  9 (9.00%) high severe
Benchmarking proof-decode/closest-descendant-merkle-value/1762: Collecting 100 samples in estimate
proof-decode/closest-descendant-merkle-value/1762
                        time:   [108.74 ns 108.94 ns 109.17 ns]
                        thrpt:  [15.032 GiB/s 15.063 GiB/s 15.091 GiB/s]
                 change:
                        time:   [-27.075% -26.565% -26.048%] (p = 0.00 < 0.05)
                        thrpt:  [+35.222% +36.176% +37.127%]
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  3 (3.00%) high mild
  1 (1.00%) high severe
Benchmarking proof-decode/closest-ancestor/1762: Collecting 100 samples in estimated 5.0010 s (13M
proof-decode/closest-ancestor/1762
                        time:   [127.16 ns 127.42 ns 127.72 ns]
                        thrpt:  [12.848 GiB/s 12.879 GiB/s 12.904 GiB/s]
                 change:
                        time:   [+24.550% +25.246% +25.930%] (p = 0.00 < 0.05)
                        thrpt:  [-20.591% -20.157% -19.711%]
                        Performance has regressed.
Found 11 outliers among 100 measurements (11.00%)
  1 (1.00%) low mild
  4 (4.00%) high mild
  6 (6.00%) high severe
Benchmarking proof-decode/decode/100299: Collecting 100 samples in estimated 7.1923 s (15k iterati
proof-decode/decode/100299
                        time:   [472.63 µs 473.45 µs 474.24 µs]
                        thrpt:  [201.70 MiB/s 202.03 MiB/s 202.38 MiB/s]
                 change:
                        time:   [-40.275% -40.096% -39.891%] (p = 0.00 < 0.05)
                        thrpt:  [+66.366% +66.934% +67.433%]
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) high mild
  1 (1.00%) high severe
Benchmarking proof-decode/get-storage-value/100299: Collecting 100 samples in estimated 5.0013 s (
proof-decode/get-storage-value/100299
                        time:   [127.21 ns 127.63 ns 128.04 ns]
                        thrpt:  [729.55 GiB/s 731.90 GiB/s 734.32 GiB/s]
                 change:
                        time:   [-63.839% -63.697% -63.569%] (p = 0.00 < 0.05)
                        thrpt:  [+174.49% +175.46% +176.54%]
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild
Benchmarking proof-decode/next-key/100299: Collecting 100 samples in estimated 5.0013 s (13M itera
proof-decode/next-key/100299
                        time:   [140.20 ns 140.80 ns 141.57 ns]
                        thrpt:  [659.80 GiB/s 663.42 GiB/s 666.28 GiB/s]
                 change:
                        time:   [-53.641% -53.349% -53.023%] (p = 0.00 < 0.05)
                        thrpt:  [+112.87% +114.36% +115.71%]
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  2 (2.00%) high mild
  11 (11.00%) high severe
Benchmarking proof-decode/closest-descendant-merkle-value/100299: Collecting 100 samples in estima
proof-decode/closest-descendant-merkle-value/100299
                        time:   [122.21 ns 122.64 ns 123.10 ns]
                        thrpt:  [758.83 GiB/s 761.67 GiB/s 764.33 GiB/s]
                 change:
                        time:   [-49.598% -49.338% -49.056%] (p = 0.00 < 0.05)
                        thrpt:  [+96.293% +97.387% +98.404%]
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  2 (2.00%) high mild
Benchmarking proof-decode/closest-ancestor/100299: Collecting 100 samples in estimated 5.0013 s (1
proof-decode/closest-ancestor/100299
                        time:   [140.15 ns 140.31 ns 140.48 ns]
                        thrpt:  [664.92 GiB/s 665.77 GiB/s 666.52 GiB/s]
                 change:
                        time:   [-11.439% -11.000% -10.593%] (p = 0.00 < 0.05)
                        thrpt:  [+11.848% +12.359% +12.916%]
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  6 (6.00%) high mild
  3 (3.00%) high severe

[tomaka]

I'm very happy with the direction of this change, plus it seems to fix some corner case situations that the missing tests didn't detect before.

While the code isn't perfect, I'm going to merge this so that I can start working on other changes that are blocked on this one.