Document RFC 2132 (copy-closures)

[RFC link](https://github.com/rust-lang/rfcs/tree/master/text/2132-copy-closures.md)

The RFC is nearing stabilization and now seems like a good time to document it. I think this is a good opportunity to explain the desugaring of closures and the like, to also fix #219.