Add mocking for generic functions (was: What features, what syntax?)

[Ferada]

This package needs improvement. In particular a good API for CLOS mocks. As always two layers, first one functions, second one macros.

Edit: After some tinkering this looks like it's adding a lot of complexity with not much of a benefit. The problem is the interaction between the features, e.g. can you rebind a regular function to a generic one, the reverse, how do you specify the method etc.

So more ideas need to be evaluated before this is viable.