Associated types are used as inputs for multidispatch

[nikomatsakis]

Currently associated types are expanded into synthetic type parameters that also lie in the same 'space' as the normal trait type parameters. This means they will be treated like inputs rather than outputs. We should fix this.

[nikomatsakis]

This is important for 1.0 primarily because we want to prevent people from doing things like implementing Deref<E=int> and Deref<E=uint> for the same type.

[nikomatsakis]

This is fixed now.

[nikomatsakis]

For posterity: it was fixed during the rewrite of associated types to not use synthetic type parameters.