8020: Power up goto_implementation r=matklad a=Veykril
by allowing it to be invoked on references of names, now showing all (trait)
implementations of the given type in all crates instead of just the defining
crate as well as including support for builtin types
![image](https://user-images.githubusercontent.com/3757771/111144403-52bb0700-8587-11eb-9205-7a2a5b8b75a3.png)
Example screenshot of `impl`s of Box in `log`, `alloc`, `std` and the current crate. Before you had to invoke it on the definition where it would only show the `impls` in `alloc`.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
Without arbitrary self types, the self type could never refer to the method type
parameters, so this wasn't a problem; but with arbitrary self types, it can.
This fixes the crash from #6668; but it doesn't make method resolution work for
these methods.