reorganise implementation directories

implementation is now repl; impl_proofs is now repl/proofs.

bootstrapping is moved to a new top-level bootstrap directory.

The bootstrapping-correctness proofs are moved to translator/repl/proofs.
They no longer depend on the actual bootstrapping; rather they talk
about "whatever the result of compile_decs oneshot is".
We might want to make an instantiated version under bootstrap/proofs, though.