Add sugar for bounds on associated types

[ftxqxd]

It would be nice if where T: Trait<AssocType: Bound> were equivalent to where T: Trait, <T as Trait>::AssocType: Bound, by analogy with T: Trait<AssocType = Type>.

[Twey]

This, by introducing an implicit type variable, has consequences for bounds on associated-type parameters of associated types, where currently one needs to bubble the type information up to the outer trait and then use a projection bound to push it back down again:

trait Foo {
    type It: Iterator<Item: Iterator<Item=u8>>;
}

currently needs to be written as

trait Foo {
  type It2: Iterator<Item=u8>;
  type It: Iterator<Item=It2>;
}

[glaebhoerl]

#2289

[Centril]

@P1start Can we close this now?

[glaebhoerl]

I thought the informal protocol was to close after an RFC has been accepted.

[Centril]

(@glaebhoerl no idea - I don't want to overstep my bounds ^,-)