auto merge of #17466 : nikomatsakis/rust/oibt, r=pcwalton

Moves the vast majority of builtin bound checking out of type contents and into the trait system.

This is a preliminary step for a lot of follow-on work:

- opt-in builtin types, obviously
- generalized where clauses, because TypeContents has this notion that a type parameter has a single set of builtin kinds, but with where clauses it depends on context
- generalized coherence, because this adds support for recursive trait selection

Unfortunately I wasn't able to completely remove Type Contents from the front-end checking in this PR. It's still used by EUV to decide what gets moved and what doesn't.

r? @pcwalton

src/liblibc/lib.rs

@@ -79,6 +79,8 @@
 #![allow(missing_doc)]
 #![allow(non_snake_case)]
 
+extern crate core;
+
 #[cfg(test)] extern crate std;
 #[cfg(test)] extern crate test;
 #[cfg(test)] extern crate native;