Named impls

As of right now all impls are anonymous and public. I propose named impls and named public impls:

impl X as Y for Z for a private impl

pub impl X as Y for Z for a public impl

This is similar to how traits currently work: you need to import them to use them.

With a non-pub impl, you can use it within your module. With a pub impl, you can use it outside your module. In any case, you need to explicitly import it with use. You can also import them from an external crate, but it follows the same rules - only available where you use them.

Later we could also add !pub impl Y for Z if we want crate-wide, anonymous impls. (Sadly, since they're already public by default, we can't have impl Y for Z and pub impl Y for Z, except maybe in Rust 2.0.)

Start by reading the proposed named-impl syntax and its comparison with trait imports in the issue. Work through the private, public, external-crate, and possible crate-wide visibility rules before proposing a design. No files or tests are named; done would require an accepted RFC-level resolution for the syntax and semantics.