Cretonne's python scripts aren't run very often, so there's little
benefit in creating .pyc files. And the .pyc files cause trouble for
some vendoring scripts. So disable them.
Give these crates each a more standard directory layout with sources in
a 'src' sub-sirectory and Cargo.toml in the top lib/foo directory.
Add license and description fields to each.
The build script for the cretonne crate now lives in
'lib/cretonne/build.rs' separating it from the normal library sources
under 'lib/cretonne/src'.
