Support for partial existentials in TAIT

[CAD97]

[playground] (Tracking issue)

With RPIT, it is possible to create a "partial existential":

fn test1<A: Iterator<Item=&'static str>>(x: A)
    -> Chain<A, impl Iterator<Item=&'static str>>
{
    x.chain(once(","))
}

With TAIT, however, this currently produces an error:

type Chained<A> = Chain<A, impl Iterator<Item=&'static str>>;
fn test2<A: Iterator<Item=&'static str>>(x: A) -> Chained<A> {
    x.chain(once(","))
}

This should probably be possible, both for concept parity for factoring unnamed RPIT out into named TAIT, and because this "partial existential" allows parameterizing partial existentials over input types in a way that allows input-type-dependent trait implementation.

[jackh726]

Works on master