Add standard corpus tests that generated Go code passes assignability checks

[sdboyer]

We should test that, for all the tests in our standard lineage corpus, the generated Go types are assignable to the schemas from which they were generated.

This'll be a bit of a nonstandard use of the corpus because we'll likely need something that writes out the generated go files to happen before running those tests, so that the generated types can be compiled into the test.

depends on #82