math/mpgen: Fix bugs in slot handling.
Several bugs, which conspired to cover their tracks.
* BaseSlot never actually stored the `omitp' and `allowp' functions.
* The MPSlot handler didn't chain up to the BaseSlot implementation
of `setup'.
* The EllipticCurveGroup's `beta' slot definition's `omitp' and `allowp'
functions used the slot name `type' instead of the object.
The incorrect lookups were hidden because the functions were never called.
The omission of `beta' values for most curve groups should then have
caused an error, only MPSlot.setup didn't chain up to the method which
would have noticed.
~mdw / catacomb / commit