Support higher-rank lifetimes in traits (e.g. <'a>Trait<'a>)

[pcwalton]

This will be needed as part of unboxed closures to complete the transition.

Nominating for P-backcompat-lang as a dependency of #14798.

[glaebhoerl]

Why only traits? :)

[glaebhoerl]

To elaborate on my drive-by comment: I'm not sufficiently familiar with the innards of the compiler to make a direct judgment, but in the abstract, I don't see why trait objects with higher-rank lifetimes should work any differently than the other candidates like structs and fns, and it may be cleaner to implement the general case than an unnecessarily specialized one.

[pnkfelix]

(I'll be putting an addendum PR on the unboxed closures RFC as a follow-up for this. -- removing I-nominated tag.)

[pnkfelix]

(actually there was already a section in the unboxed closures RFC saying that we need a separate RFC to describe the need for trait references like <'a> Fn<(&'a A), &'a B>)