gom: Fix fallout from object-types leak fix

The fix in 54737fd6c99fb20dc6c8877e035740b9a206258f was incomplete
and 2 test cases were crashing during migration.

Furthermore, the automatic migrator was freeing the list of object types
after migrating to version 1, making it crash for any migration past
that.
gom: API documentation improvements
0.2.0