Implement UFCS (Tracking RFC 132)

[huonw]

https://github.com/rust-lang/rfcs/blob/master/text/0132-ufcs.md

[pnkfelix]

This is (expected to be a) backwards-compatible change, so while we do desire it, it is not a strict necessity for 1.0

Assigning P-high, not 1.0 milestone.

[nrc]

Question: in a trait or impl, should it be possible to call foo(x) where foo is a method in scope? We don't allow calling foo() (i.e., with an implicit self), but we do allow calling bar where bar is a function in scope. It seems to me like we should allow it since it is weird to need qualification of an item in an inner scope but not an outer one. OTOH, it means we would get worse error messages where a programmer has omitted an explicit self, since it would just be a "not enough args" error, rather than something more specific.