Support object types as values.

Fixes #7.
Inlined `self.iterkeys` invocation in `__dealloc__`.

Removed extra `Py_XINCREF` in `_fromvalue`.
Demote part of docstring to comment and remove duplication.
Drop unused variable.
Use `hattrie_iter_val` directly when destroying values.
Fix segmentation fault.
Drop some more unused variables.
Add leak test.
Fix misuse of `is` in tests.
Merge pull request #8 from b4hand/support-object-values

Support object types as values.