Create CounterResult for BinaryCollection increment/decrement operations

All BinaryCollection operations return a MutationResult. Only append/prepend should return a MutationResult while increment/decrement should return CounterResult.

The C bindings have all counter operations returning a pycbc_ValueResult. Therefore mutation tokens will be missing for the increment/decrement operations. This should be updated to comply with the RFC.

TODO:

1. Add CounterResult object
2. Have BinaryCollection increment/decrement return a CounterResult object
3. Update C bindings to include mutation tokens for append/prepend counter operations