Improve interpolation of macro lhs and doc comments. Remove Matchers

[pczarn]

Closes#14197

Removes the matchers nonterminal.

If you're using $foo:matchers in a macro, write $foo:tt instead.

[breaking-change]

[huonw]

Does this handle the following?

// other, so the result of the whole thing should be "let z_123 = 3; z_123macro_rules! g (($x:ident) =>
({macro_rules! f(($y:ident)=>({let $y=3;$x}));
f!($x)}))fna(){g!(z)}

(If not, we probably shouldn't close #6994.)

[pczarn]

@huonw no, it fails to compile with an unresolved name. Whoa, it seems those are two different idents.

OK, this is just preliminary work for #6994, matcher quotation: #17396, doc comments: #14197, and some further improvements I have in mind.

r?

[pnkfelix]

cc me (I will try to review on Monday if no one else gets to it in the meantime).

[pnkfelix]

@pczarn (okay, I'll try to avoid making comments about things that were addressed in later commits ... )

[alexcrichton]

Feel free to ping the PR whenever you force-push it, sadly github doesn't send out notifications for that kind of event!

[adrientetar]

@alexcrichton Look like it was updated

[pczarn]

Yes, and this update actually works. I didn't have time to find mistakes after rebasing, previously