libsyntax: Remove `~self` and `mut ~self` from the language.

This eliminates the last vestige of the `~` syntax.

Instead of `~self`, write `self: Box<TypeOfSelf>`; instead of `mut
~self`, write `mut self: Box<TypeOfSelf>`, replacing `TypeOfSelf` with
the self-type parameter as specified in the implementation.

Closes #13885.

[breaking-change]

r? @alexcrichton