Allow mut qualification on idents in patterns

[huonw]

e.g.

let (mut x, mut y) = ...;

match p {
    Foo(mut q) => { ... }
    mut s @ Baz(*) => { ... }
}

fn foo((mut a, mut b): (~str, ~[float])) { ... }

let f = |mut x| { ... };

The grammar would become approximately

ident_pat = "ref"? "mut"? ident
pat = "_" 
     | ident_pat
     | path "(" "*" ")"
     | path "(" pat* ")"
     | path "{" [ident ":" pat | ident^]* "}"
     | ident_pat "@" pat

(The ident marked ^ would become a ident_pat to solve #6137.)

This would also mean that the grammar for let could be simplified to just "let" pat "=" expr ";" (rather than "let" "mut"? pat "=" expr ";" as it currently is), and similarly for function arguments.

(I think this has been thrown about for a while, but I can't find an issue; and I very briefly experimented with this, but quite a few places seem to assume that only let and function args allow mut.)

[chris-morgan]

Would this fix mut self and mut ~self?

[huonw]

They're not pattern contexts (yet?), so no.

[brson]

Nominating.

[luqmana]

From #rust-internals:

[08:28:31 pm] <Luqman> brson: was there any talk of mut on ident patterns in the meeting? ( https://github.com/mozilla/rust/issues/9792 )
[08:30:21 pm] <brson> Luqman: yes, I believe we want it
[08:30:26 pm] <brson> pcwalton: can you confirm?
[08:31:04 pm] <Luqman> sweet, once i clean up my branch later i'll send a pull
[08:39:29 pm] <pcwalton> brson: we didn't really talk about it at the meeting but I'm totally in favor and I think everyone is