improve tests for version parser (#696)
The high level goal here is to improve the tests for the version parser.
Namely, we now check not just that version strings parse successfully,
but that they parse to the expected result.
We also do a few other cleanups. Most notably, `Version` is now an
opaque type so that we can more easily change its representation going
forward.
Reviewing commit-by-commit is suggested. :-)
