Allow trait methods to return impl Trait types.

[kevincox]

It should be possible for methods in traits to have a return value specified as impl Trait. This would allow traits to return values without having to specify the exact type or wrap in indirection.

This would obviously not be object-safe as the size of the return value would not be known.

[jonas-schievink]

Please refer to https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md#limitation-to-freeinherent-functions for why this isn't allowed. Doing this requires substantial language design efforts and an RFC, so closing in favor of that.