Support `T::Type` shorthand for referencing associated types

[nikomatsakis]

Today we support only the very explicit notation of <T as Trait>::Type. It'd be nice to support the shorter notation of T::Type. Eventually this should work in as wide a variety of circumstances as possible, but for the time being it might be ok to introduce a hack to make this work, such as only if T has a single applicable trait bound. But we have to be careful that this hack is forwards compatible (so, e.g., just picking the first trait bound unambiguously is probably not a good idea).

[milibopp]

In the definition of a trait with associated types, would it even be possible to refer to the associated type by its name only (i.e. without the <Self as Trait>:: prefix altogether)?