Relative paths in Rust 2018

[petrochenkov]

@SimonSapin

This looks like a breaking change, since such paths in 2015 are equivalent to the proposed crate:: (they can refer to any kind of item at the root of the crate). Not having breaking changes was a big selling point of the previous proposals.

Huh? The previous proposal (which is currently implemented) implies a huge breaking change on edition switch (but no breakage without edition switch), look at any "2015 -> 2018" transition PR.
This proposal seems to keep the same approach.

EDIT: ::something already means extern::something in edition 2018 in the current implementation.