Update grammar for parser unification.

[ehuss]

Updates the grammar for function and associated item unification/simplification. Over a series of PRs, various parts of the grammar were unified so that some the same grammar is reused in various places, with restrictions and validation applied after parsing.

Some of the PRs:

• Merge TraitItem & ImplItem into AssocItem` rust#67131: Merge TraitItem & ImplItem into AssocItem
• parse: merge fn syntax + cleanup item parsing rust#68728: merge fn syntax + cleanup item parsing
• parser: syntactically allow self in all fn contexts rust#68764: syntactically allow self in all fn contexts
• parse: towards unified fn grammar rust#68788: towards unified fn grammar
• parse: unify function front matter parsing rust#69023: unify function front matter parsing
• parse: fuse associated and extern items up to defaultness rust#69194: fuse associated and extern items up to defaultness

This also includes a few other fixes:

• Function front matter fixes (like async const).
• Fix function pointer front matter (which does not syntactically allow async or const).
• Add some notes about various restrictions.

I can provide links to rustc parser functions if you want, but I think they are pretty easy to find.

Closes #750
Closes #751

[Havvy]

I did a quick read through the changes, and they look good to me. That said, I haven't checked they match the compiler, although I have no doubts they all do, so I shall withhold my official review until tomorrow when I am rested.

[Havvy]

Approved. PR now conflicts with master.

[ehuss]

Thanks! I wish GitHub would notify when conflicts arise.