Unit tests for typestate operations

[catamorphism]

If typestate ends up getting kept around, it would be good to have some unit tests for operations such as the ones in tstate::ann. There are some bits of the theory that I never quite worked out (like figuring out which of two different reasonable models for what the trit vector operations mean is the one we're using) and having test cases would be one way to inject a bit more rigor into the scene.

Contingent on typestate not being removed.

[nikomatsakis]

Not relevant as of commit 41a21f0, which removed current incarnation of typestate.