This PR announces the feature freeze period talked about in
https://github.com/rust-lang/rust-clippy/issues/14364
1. Add the page to the book
2. Modify the in-Github templates.
3. The third commit (to be squashed into the first) rolls the date
mentioned 6 weeks (so, starting on May 9th and ending on August 20th).
This gives us a comfortable buffer to make choices for this period.
We have a pending discussion on the #14364. So having some more time to
make choices is very nice. I'm also preparing a Github action for
detecting new lints and posting a comment about the feature freeze
(being worked on in another branch)
Something I'd like comment on is the date formatting. I'm not sure if
"May 9th to the first of August" is the correct way of writing this
information in a book 😅.
changelog: Announce the feature freeze from May 9th to the first of
August
`rustc_middle` is a huge crate and it's always good to move stuff out of
it. There are lots of similar methods already on `Span`, so these two
functions, `in_external_macro` and `is_from_async_await`, fit right in.
The diff is big because `in_external_macro` is used a lot by clippy
lints.
