Simulate biggroup `batch_mul` in general

[codygunton]

At the moment, we only simulate biggroup batch muls in the case when Fr is a field_t (in fact this restriction is imposed in a way the relies on loose coupling, since we single out the field_t case by imposing a constraint on the G type). t would be more work to handle the case when Fr is a bigfield instance since we don't have a uniform interface; in that case, get_value returns a uint512_t.