Remove all public reexports

They're leftovers from the enum namespacing transition

Other than removing public reexports, I also had to publicize a type
(compile::InstIdx). It should have always been public but no
error was ever given because of this bug:

https://github.com/rust-lang/rust/issues/6477

I also had to enable documentation for compile::Ist because it was
complaining that there wasn't any.