Remove some unused/unnecessary code.
Refactor collection → subcollection finding.
Fix debilitating bugs with non-polymorphic serializers.
Add non-polymorphic tests.

This is not complete yet, and one of the tests is failing, indicating an
unsoundness in the technique used (a serializer without ``fields``
specified). This will need to be fixed eventually.
Make cqrs.tests import all submodules.

This means that individual tests can be run without breaking.