ec.c, field.c, group.c, mp.c: Fix conversions in `pow' and scalar mul.

ec.c, field.c, group.c, mp.c: Fix conversions in `pow' and scalar mul.

The base and modulus (if present) should belong to the base ring; the
exponent or scalar should be an integer; and we should use implicit-
conversion rules for all three.  (Of course, the modulus can also be
`None'.)

This tightening temporarily breaks a test multiplying an elliptic curve
point by a prime-field element Q n.  That used to work, but not for any
especially principled reason; and this wasn't commutative (n Q would
multiply n by the x-coordinate of Q coerced into the scalar field).  We
shall fix this soon.