Add benchmarks for the whole PRE scenario and some internals

[fjarri]

Fixes #9

• added benchmarks for Capsule::from_public_key(), Capsule::open_original(), and Capsule::open_reencrypted()
• internal benchmarks are only run with the feature bench-internals
• added benchmarks for encrypt(), decrypt_original(), generate_kfrags(), reencrypt(), and open_reencrypted() (only for 2 out of 3 scenarios)
• added benchmarking to Github actions (build only, running them is too slow)

[codecov-commenter]

Codecov Report

Merging #54 (844d376) into master (55af276) will decrease coverage by 0.62%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   84.56%   83.94%   -0.63%     
==========================================
  Files          12       13       +1     
  Lines         946      953       +7     
==========================================
  Hits          800      800              
- Misses        146      153       +7     

Impacted Files	Coverage Δ
umbral-pre/src/bench.rs	0.00% <0.00%> (ø)
umbral-pre/src/capsule.rs	89.34% <ø> (ø)

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[cygnusv]

Can we see the benchmark reports on CI? I can't find them

[fjarri]

Not currently, the CI task added in the PR only builds the benchmarks, but does not run them.

Does it make sense to actually run them in CI? It's pretty slow, and in order to get some use out of it we need to at least check for performance regressions automatically - I wonder if it's possible.