Initial version of "`Compose` trait" RFC

Rendered: https://github.com/apoelstra/rfcs/blob/compose-trait/active/0000-compose-trait.md

cc https://github.com/rust-lang/rust/issues/16541
`Compose` could be used on an `Iterator`, as a synonym of the `chain` method. It could also be used on `GenericPath`, performing a `join`.