The List and Dict types are no longer implicitly available. They must be
imported from typing.
Type annotations must appear before the doc comment in a function. Also
fix type errors in these functions that weren't detected before.
The 'lib/cretonne' directory will be the new root of a stand-alone
cretonne crate containg both Python and Rust sources.
This is in preparation for publishing crates on crates.io.
