Position independent operations on different types

[jaromil]

Overloaded operators like * that take different types (ECP * BIG) should check for the position, for instance, right now ECP multiplication needs to have the BIG number always as the second argument, else an error is returned. Instead of returning an error, we should check the type of both arguments and execute the right operation. In this case, only BIGBIG or ECPBIG is possible, errors would still be returned for ECP*ECP.