test: 'ratcher' is not a word
std: mark two helper functions #[inline].

`str::utf8_char_width` and `char::from_u32` are tiny, which means it's a
big performance hit to call them in a tight loop outside libstd.
std: Remove some nonsense from old std::io docs

Most of this stuff is irrelevant implementation notes from last year.
This trims out the stuff that isn't appropriate for user-facing docs.
Fix C function FFI example in the Rust cheatsheet
Tutorial: fix typo
Merge remote-tracking branch 'kud1ing/patch-1'
Merge remote-tracking branch 'tbu/pr_doc_smallfix'
Merge remote-tracking branch 'brson/iodoc'
Merge remote-tracking branch 'brson/ratcher'
Merge remote-tracking branch 'huonw/inline-helpers'
auto merge of #12509 : brson/rust/rollup, r=brson

Rollup of https://github.com/mozilla/rust/pull/12506, https://github.com/mozilla/rust/pull/12500, https://github.com/mozilla/rust/pull/12480, https://github.com/mozilla/rust/pull/12478, https://github.com/mozilla/rust/pull/12461.