Rollup merge of rust-lang#86557 - ehuss:update-books, r=ehuss

Update books

## nomicon

10 commits in 55de6fa3c1f331774da19472c9ee57d2ae9eb039..b9ca313e687c991223e23e5520529815dc281205
2021-05-12 00:31:01 +0900 to 2021-06-22 12:02:20 -0400
- The #[repr(C)] attribute on the callback example is not necessary, since the type is not used in C.
- Reorganize some chapters (rust-lang/nomicon#282)
- Mention "extern types" on the opaque structs section (rust-lang/nomicon#273)
- Clarify the conditions on the aliasing section (rust-lang/nomicon#272)
- Upgrade to edition 2018 (rust-lang/nomicon#280)
- Update some wording making reference to issues/RFCs (rust-lang/nomicon#271)
- Some improvements on the "subtyping" chapter (rust-lang/nomicon#278)
- Clarify casting between the same size fixed ints (rust-lang/nomicon#277)
- Add a link to show why unused lifetimes on structs are forbidden (rust-lang/nomicon#276)
- Fix small typo in the Drop Check chapter (rust-lang/nomicon#275)

## reference

8 commits in 8f598e2af6c25b4a7ee88ef6a8196d9b8ea50ca8..d9699fa8f3186440fdaadd703d63d8d42322c176
2021-06-01 19:00:46 +0100 to 2021-06-21 12:23:10 -0700
- Make explicit reference to scrutinee expression in grammar snippet (rust-lang/reference#1044)
- Document sub-namespaces. (rust-lang/reference#1043)
- Default all examples to 2018 edition. (rust-lang/reference#1041)
- Minor update to macros. (rust-lang/reference#1048)
-  (rust-lang/reference#1049)
- Add a note why the same size int casting is a no-op (rust-lang/reference#1046)
- Add notes on `#[target_feature]` for wasm (rust-lang/reference#1047)
- Make statement about variable visibility more precise (rust-lang/reference#1045)

## rustc-dev-guide

8 commits in c8da5bfd1c7c71d90ef1646f5e0a9f6609d5c78a..fe34beddb41dea5cb891032512a8d5b842b99696
2021-06-04 09:08:56 +0200 to 2021-06-21 21:50:12 +0200
- Update "Inference variables" section (rust-lang/rustc-dev-guide#1145)
- Document how to run unit tests (rust-lang/rustc-dev-guide#1141)
- We stopped using allow_internal_unstable a while ago (rust-lang/rustc-dev-guide#1142)
- Change the feature used as an example of stabilizing lib features (rust-lang/rustc-dev-guide#1143)
- We use HIR to do type inference, trait solving and type checking (rust-lang/rustc-dev-guide#1139)
- Add suggested settings note for coc (rust-lang/rustc-dev-guide#1144)
- move 7/8 to prose
- Add a section on keeping things up to date in the git section

## edition-guide

2 commits in 302a115e8f71876dfc884aebb0ca5ccb02b8a962..c74b2a0d6bf55774cf15d69f05dfe05408b8f81a
2021-05-21 10:46:11 -0400 to 2021-06-14 10:48:27 -0700
- Fix lies. (rust-lang/edition-guide#244)
- Add short summaries to all 2021 edition changes. (rust-lang/edition-guide#243)

## embedded-book

1 commits in 7349d173fa28a0bb834cf0264a05286620ef0923..cbec77fbd8eea0c13e390dd9eded1ae200e811d1
2021-05-25 13:59:05 +0000 to 2021-06-10 06:26:32 +0000
- Fix punctuation in c-with-rust.md  (rust-embedded/book#294)