Rollup merge of rust-lang#69641 - ehuss:update-books, r=ehuss

Update books

## nomicon

8 commits in 3e6e1001dc6e095dbd5c88005e80969f60e384e1..9f797e65e6bcc79419975b17aff8e21c9adc039f
2019-12-14 22:08:52 +0000 to 2020-03-04 03:45:13 +0100
- Rustup to latest rustc (rust-lang/nomicon#203)
- (minor) Add backticks around type names (rust-lang/nomicon#197)
- Add book.toml (rust-lang/nomicon#185)
- Rename `Alloc` to `AllocRef` (rust-lang/nomicon#188)
- Lifetimes: explain how to fix destructor example (rust-lang/nomicon#195)
- mention soundness (rust-lang/nomicon#194)
- Fix example in FFI section Calling Rust from C (rust-lang/nomicon#193)
- Removed repeated word (rust-lang/nomicon#191)

## reference

8 commits in 64239df..559e09c
2020-02-10 19:05:13 +0100 to 2020-03-02 01:17:14 +0100
- Syntax error fix (rust-lang/reference#769)
- Fix incorrect pseudocode for #[repr(C)] struct alignment (rust-lang/reference#766)
- Replace "Field-Less" with "Fieldless" (rust-lang/reference#768)
- Removed repeated word (rust-lang/reference#767)
- Update for change in const lint name. (rust-lang/reference#764)
- semantic type -> resolved type (rust-lang/reference#761)
- add behavior change of relative paths without `self` in 2018 edition (rust-lang/reference#757)
- assignment operator expressions -> compound assignment expressions (rust-lang/reference#759)

## rust-by-example

3 commits in 32facd5522ddbbf37baf01e4e4b6562bc55c071a..db57f899ea2a56a544c8d280cbf033438666273d
2020-02-11 09:25:06 -0300 to 2020-02-18 17:46:46 -0300
- Minor typo fix in src/mod/visibility (rust-lang/rust-by-example#1309)
- Don't suggest Into implements a reverse conversion (rust-lang/rust-by-example#1307)
- Improve grammar in example of 'in let' section (rust-lang/rust-by-example#1308)

## embedded-book

2 commits in b2e1092bf67bd4d7686c4553f186edbb7f5f92db..b81ffb7a6f4c5aaed92786e770e99db116aa4ebd
2020-01-30 08:45:46 +0000 to 2020-02-27 08:06:04 +0000
- Setting output to `high` needs a `true` argument  (rust-embedded/book#227)
- Add licence notes to index.md  (rust-embedded/book#226)

Author: Dylan-DPC