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]
Add tests that the injected runtime is inaccessible
auto merge of #15987 : brson/rust/hidestdrt, r=alexcrichton

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