rustdoc: Suck in all impls from external crates

rustdoc: Suck in all impls from external crates

There is currently no way to query all impls for a type from an external crate,
and with primitive types in play this is also quite difficult. Instead of
filtering, just suck in all impls from upstream crates into the local AST, and
have them get stripped later.

This will allow population of all implementations of traits for primitive types,
as well as filling in some corner cases with inlining documentation in other
cases.