Rollup merge of rust-lang#84668 - ehuss:update-books, r=ehuss

Update books

## reference

5 commits in e1abb17cd94cd5a8a374b48e1bc8134a2208ed48..d23f9da8469617e6c81121d9fd123443df70595d
2021-04-07 08:09:48 -0700 to 2021-04-28 11:16:44 -0700
- Document or-patterns (rust-lang/reference#957)
- fixed a typo in traits.md (rust-lang/reference#1009)
- Improve clarity and style consistency of crate type list (rust-lang/reference#1005)
- added macro_rules to weak keywords (rust-lang/reference#1008)
- Move non-ascii-idents content from unstable book to reference. (rust-lang/reference#999)

## book

1 commits in b54090a99ec7c4b46a5203a9c927fdbc311bb1f5..50dd06cb71beb27fdc0eebade5509cdcc1f821ed
2021-03-24 11:21:46 -0500 to 2021-04-23 13:21:54 -0500
- Update link in COPYRIGHT (http to https) (rust-lang/book#2704)

## rust-by-example

3 commits in c80f0b09fc15b9251825343be910c08531938ab2..e0a721f5202e6d9bec0aff99f10e44480c0da9e7
2021-04-08 10:28:17 -0300 to 2021-04-27 09:32:15 -0300
- broken long comments in src/types/cast.md to several shortones (rust-lang/rust-by-example#1430)
- Fix link of formatting traits (rust-lang/rust-by-example#1410)
- chore: Fix the indention of Borrowed definition (rust-lang/rust-by-example#1436)

## rustc-dev-guide

8 commits in a9bd2bb..e72b43a
2021-04-09 18:12:21 -0400 to 2021-04-27 12:35:37 -0700
- Suggest using `git range-diff` (rust-lang/rustc-dev-guide#1092)
- Remove the possible unnecessary flag
- Replace some Travis-related things completely
- Trigger GHA only on the original repo
- Add sample nix shell
- more RA config suggestions (rust-lang/rustc-dev-guide#1114)
- Add Polymorphisation paper (rust-lang/rustc-dev-guide#1093)
- Mention unpretty=mir-cfg for debugging MIR