Pass `Checker` by immutable reference to lint rules (#16012)
This very large PR changes the field `.diagnostics` in the `Checker`
from a `Vec<Diagnostic>` to a `RefCell<Vec<Diagnostic>>`, adds methods
to push new diagnostics to this cell, and then removes unnecessary
mutability throughout all of our lint rule implementations.
Consequently, the compiler may now enforce what was, till now, the
_convention_ that the only changes to the `Checker` that can happen
during a lint are the addition of diagnostics[^1].
The PR is best reviewed commit-by-commit. I have tried to keep the large
commits limited to "bulk actions that you can easily see are performing
the same find/replace on a large number of files", and separate anything
ad-hoc or with larger diffs. Please let me know if there's anything else
I can do to make this easier to review!
Many thanks to [`ast-grep`](https://github.com/ast-grep/ast-grep),
[`helix`](https://github.com/helix-editor/helix), and good ol'
fashioned`git` magic, without which this PR would have taken the rest of
my natural life.
[^1]: And randomly also the seen variables violating `flake8-bugbear`?
