makes sense of implicit macros! now in master

This is a port of https://github.com/scala/scala/pull/2494 from 2.10.x. Depending on your preference we can either get it in 2.11.0-M3, since it's already been reviewed and merged in 2.10.2, or we can delay it until 2.11.0-M4.

review @paulp @retronym