Rollup merge of rust-lang#76255 - ehuss:update-books, r=ehuss

Update books

## reference

4 commits in 1b6c4b0afab97c0230433466c97167bbbe8445f6..25391dba46262f882fa846beefaff54a966a8fa5
2020-08-18 17:04:28 -0700 to 2020-09-02 07:22:55 -0700
- clarify when reading uninititalized memory is allowed (rust-lang/reference#852)
- Update patterns chapter, add rest patterns. (rust-lang/reference#876)
- Improve Type-Coersion Documentation (rust-lang/reference#843)
- Added variable back into example. (rust-lang/reference#880)

## book

3 commits in c0a6a61b8205da14ac955425f74258ffd8ee065d..e5ed97128302d5fa45dbac0e64426bc7649a558c
2020-08-14 14:21:49 -0500 to 2020-08-31 12:53:40 -0500
- Fix type mismatch in listing 10-5 (rust-lang/book#2441)
- Update ppendix-06-translation.md (rust-lang/book#2437)
- Correct no-listing-10-result-in-tests: Take tests module out of the main function (rust-lang/book#2430)

## rust-by-example

3 commits in 80a10e22140e28392b99d24ed02f4c6d8cb770a0..19f0a0372af497b34369cf182d9d16156cab2969
2020-08-08 09:56:46 -0300 to 2020-08-26 09:38:48 -0300
- prefer `length` over `size` when talking about number of elements vs. bytesize (rust-lang/rust-by-example#1372)
- Split out variable shadowing into a separate example (rust-lang/rust-by-example#1370)
- Update extern crate related sections (rust-lang/rust-by-example#1369)

## edition-guide

1 commits in bd6e4a9f59c5c1545f572266af77f5c7a5bad6d1..81f16863014de60b53de401d71ff904d163ee030
2020-07-12 17:37:08 -0500 to 2020-08-27 13:56:31 -0700
- Fix a small typo. (rust-lang/edition-guide#218)