implemented the c_code for the scalar mod op. It correctly implement the python semantic of mod on x86.

better implementation of scalar.Composite.__eq__ and .__hash__. started the implementation of Composite pickling(disabled).

added scalar.Composite.__str__ as we don't want all element of the op to happear in the str representation.

better test for the canonizer. skip test that are know to fail because they are not implemented. They are not real error or failure.

changed CLinker.cmodule_key to do the real computation in a static method to allow calling it from elsewhere.

CLinker returns keys that include the role of each variable, as well as the
no_recycling parameter.

This fixes an incorrect result I am seeing, and I believe it fixes the
mysterious 'output-borrowing' behaviour that Olivier Delalleau reported.

FAST_RUN to the behaviour of profile mode.