Schema Optimisation Bug Fixes

Fix two bugs introduced in the schema optimisations, and identified in issue #118.

Also taken the unit tests for Schema from https://github.com/keleshev/schema/blob/master/test_schema.py and added them under the schema vendor package.    The file `hai` is required by the unittests - unfortunately it doesn't have a setup/teardown.  I have been converted these tests from `pytest` to Python's native `unittest`.  

Happily, these tests pick up both the errors from issue #118 and now all pass!

I have not hooked these tests up to the `rez selftest` command, but I think that might be a good idea (perhaps `rez selftest --vendor`)?