Unboxed closures

A relatively complete description of by reference closures that aims to encompass entirety of current thinking, except for bound lifetimes.

cc @pcwalton
Is this talking about a closure body? How is `|int| -> uint` even relevant?
I feel that the idea presented in [this #97 comment](https://github.com/rust-lang/rfcs/pull/97#issuecomment-44680119) (excluding trait inheritance) has been ignored/rejected too hastily.
If "inference" here refers to determining only one trait a closure body represents, then it's not even the same thing.