9eb0bab9de
This PR implements rust-lang/rfcs#1023. In the process it fixes #23086 and #23516. A few impls in libcore had to be updated, but the impact is generally pretty minimal. Most of the fallout is in the tests that probed the limits of today's coherence. I tested and we were able to build the most popular crates along with iron (modulo errors around errors being sendable). Fixes #23918.