RFC: `self` in `use`

Proposes the `mod` keyword used to refer the immediate parent namespace in `use` items (`use a::b::{mod, c}`) to be changed to `self`.

[Rendered](https://github.com/lifthrasiir/rust-rfcs/blob/496ea64a03c841079b6caebc772e627d84dabdfe/text/0000-self-in-use.md)