This makes it a little simpler to generate 'match' statements, and
it performs deduplication of identical arms. And it means I don't
have to think about as many strings like '{} {{ {}.. }} => {}'
when I'm trying to think about how instructions work :-).
The main change is that it avoids creating blank lines when processing
docstrings.
This also adds blank lines in various places to make the generated code
prettier.
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.
