There's 4 classes of variables, depending on whether they have defs in
the source and destination patterns.
Add more XForm verification: In a legalize XForm, all source defs must
be outputs.
Fix a legalize pattern bug caught by this.
The check.sh script always runs the Python unittests with 'python', but
if 'python3' is in the path, run it with that too.
Fix a Python 3 compat issue and avoid passing None to max() and min().
Use an explicit intersect() function instead to intersect intervals.
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.
