move most of front to libsyntax

I tried to move all of front to libsyntax but feature_gate.rs and std_inject.rs have small but annoying dependencies on other rustc stuff. So I'm not really sure if moving just this stuff is worthwhile, but it does seem a bit of a better fit in libsyntax.

r?
I would move the functionality of this entire file to `ast_map`, maybe `Map::from_crate`?.