Closure kind inference: infer whether a closure is `Fn`, `FnMut`, etc

[pcwalton]

Updated description

Closure kind inference: infer whether a closure is Fn, FnMut, etc

Original Description

Fn(int) -> int, etc.

Nominating for 1.0, P-backcompat-lang.

[pnkfelix]

P-backcompat-lang, 1.0.