Push BangType down into Type. Groundwork for #11.
Introduce type equality constraint constructor
Fix ConstraintKinds tests

We now support type equality constraint
Add tests for the PackageImports extension.

Include positive (works when PackageImports is present) and negative
(compile error if PackageImports is absent) tests.
No code HSE code changes required to make these tests pass.
Test: use forward slash instead of a platform-specific. See #142