Unboxed closures

A relatively complete description of by reference closures that aims to encompass entirety of current thinking, except for bound lifetimes.

cc @pcwalton
On Wed, Jun 11, 2014 at 03:37:11PM -0700, Chris Morgan wrote:
> This (and the next couple of example functions) should be `Fn<(int,), int>` rather than `Fn<int, int>`, I think?

Sadly yes. Thanks.