First pass at proper overloading support

This still could use some more tests, but the role application bug is fixed,
and all other role-related tests seem to be passing, so yay.
Add/update basic tests for CMOP::Overload class
Make sure that overloads are cloned properly during role composition
Update docs for overloading APIs
Die if an exception class has no metaclass
Add Changes & Delta entries for overloading changes