Implement syntax for anonymous unboxed closures

[pcwalton]

This is part of unboxed closures. cc @nikomatsakis

I suggest that we adopt the following syntax for now. We can change it and bikeshed it later.

|&: x, y| x + y // Fn
|&mut: x, y| x + y // FnMut (the commonest, eventually the default)
|: x, y| x + y // FnOnce

These create anonymous struct types that move their upvars (cf. #12831) and implement the Fn/FnMut/FnOnce traits.

Per discussion last week I believe that this is necessary for 1.0. Nominating for backcompat-lang because we will remove the old closures after this is done.

[brson]

1.0 backcompat lang, but please get an RFC approved.

[pcwalton]

This is nearly done. Once Fn and FnOnce are implemented, and some better test coverage for the entire end-to-end feature (both cross-crate and intra-crate) is in, I'll close this.

[brson]

Tracking rust-lang/rfcs#114