Simplify rustc_public context handling

We no longer need two thread-local variables to store the context.
We used to have two because the conversion logic used to live in a
separate crate from the rest of the business logic.

I'm also removing the Container struct and replacing the
CompilerInterface trait with a struct. This removes the unnecessary
indirection and code duplication. Using a trait would also block us
from adding any generic method to the compiler interface.

Author: celinval