RFC for attributes on match arms and statements.
Revise after splitting #8 for match arm attributes (now landed).
Revisions to include expressions more definitely.

`if` is annoying.
An unfortunately placed # broke the formatting.
Reword a point for clarity.
Feedback from @pnkfelix's review:

- explicitly disallowing attributes on `if`/`else` due to weirdness
- provide alternatives for this
- add a drawbacks section discussing syntactic lock-down and the
  antipattern of `#[cfg]`
- clarify that this RFC is just for parsing attributes, not requiring that
  `#[cfg]` stripping be added in all these new places.
Finish my

sentences.
Merge remote-tracking branch 'huonw/attributes'
RFC 40 adds more valid attribute locations