Rollup merge of rust-lang#81399 - ehuss:update-books, r=ehuss

Update books

## nomicon

7 commits in a8584998eacdea7106a1dfafcbf6c1c06fcdf925..bbf06ad39d1f45654047e9596b750cc6e6d1b693
2021-01-06 12:49:49 -0500 to 2021-01-22 07:07:31 -0800
- Fix alloc link in exotic-sizes for local docs (rust-lang/nomicon#255)
- Remove TODO
- Fix small punctuation error
- Arc revisions (Clone atomic explanation) (pt2/3(+?))
- Fix Arc Clone
- Arc revisions (pt1/2(+?))
- Simple Arc implementation (without Weak refs)

## reference

5 commits in 50af691f838937c300b47812d0507c6d88c14f97..f02b09eb6e8af340ad1256a54adb7aae2ff3163e
2021-01-12 21:19:20 -0800 to 2021-01-22 01:53:02 -0800
- Fix missing space (rust-lang/reference#941)
- Start documenting name resolution. (rust-lang/reference#937)
- Fix plural and delete spurious words in comparison ops (rust-lang/reference#932)
- Document execution order (rust-lang/reference#888)
- Compound operator expressions (rust-lang/reference#915)

## book

3 commits in ac57a0ddd23d173b26731ccf939f3ba729753275..e724bd826580ff95df48a8533af7dec1080693d4
2021-01-09 14:18:45 -0500 to 2021-01-20 08:19:49 -0600
- Fixes rust-lang/book#2417. Get the index from user input instead of a const. (rust-lang/book#2566)
- Turn off the playground in a bunch more lib.rs inclusions (rust-lang/book#2569)
- Merge pull request rust-lang/book#2567 from rust-lang/rust-1.49

## rust-by-example

1 commits in 03e23af01f0b4f83a3a513da280e1ca92587f2ec..f633769acef68574427a6fae6c06f13bc2199573
2021-01-09 10:20:28 -0300 to 2021-01-13 20:58:25 -0300
- Fixed styling on closure example (rust-lang/rust-by-example#1405)