RFC: `self` in `use`

This proposes the `mod` keyword used to refer the immediate parent namespace
in `use` items (`use a::b::{mod, c}`) to be changed to `self`.
Merge branch 'self-in-use' of https://github.com/lifthrasiir/rust-rfcs into lifthrasiir-self-in-use
Merge branch 'lifthrasiir-self-in-use'