Allow importing assoicated methods

[josh]

@tvallotton

Is there a reason not to allow importing associated methods from types and traits? Enums support importing constructors, which is similar. It would be nice to be able to do stuff like:

use Default::default; 
use std::MaybeUninit::uninit;

No reason whatsoever, except that it touches name resolution and hasn't been done yet, and there are relatively few people capable of doing it. (@petrochenkov being one of them, and thanks @CAD97 for the link.)

We should absolutely support this.