Native types should have crate-relative identifiers

[pcwalton]

Native type identifiers are global, not crate-relative. This doesn't seem easy to typecheck because of the diamond import problem:

Suppose:

• Crate A defines a native type foo;
• Crate B imports A and exports A.foo;
• Crate C imports A and exports A.foo;
• Crate D imports B and C and unifies B.foo and C.foo.

B.foo and C.foo won't be considered compatible.

Instead of having one global value set, we could subject them to crate-relative name resolution, just like any other type. In other words, we would have e.g. "TY_native of crate_id * native_id".

[graydon]

WONTFIX (not required for bootstrapping; code in rustc is all different. defs are all crate-relative there).