rustc: Future proof runtime injection

Rename and gensym the runtime on import, so that users
can't refer to the `native` crate.

This is unlikely to break code, but users should import the "native" crate directly.

[breaking-change]

cc @alexcrichton
FWIW, a `use` of a crate at the top level like this fails already: http://is.gd/MfQbdO (swapping the comments there allows it to compile.)

https://github.com/rust-lang/rust/issues/13968