Bug: type aliases don't work with --remove-associated-types

[Nadrieril]

Type aliases in rust are glorified macros: they are not checked for well-formedness and as such don't require the proper trait clauses to be in scope:

trait Trait { type Output; }
type Type<F> = <F as Trait>::Output;

This already requires hack today, but with --remove-associated-types it's worse. Solutions include:

• Putting type aliases behind an option and error if both options are on;
• Add the missing trait clauses automatically somehow.