ec.c, field.c, group.c: Allow exponents and scalars from prime fields.

ec.c, field.c, group.c: Allow exponents and scalars from prime fields.

The idea is that the unit group of a finite field, and the point group
of an elliptic curve, can usefully be viewed as modules over a suitable
integer quotient ring; and, even more so, prime-order subgroups of these
can be considered as one-dimensional vector spaces over prime-order
fields.  It's therefore helpful to allow elements of the appropriate
field as exponents or scalar multipliers as appropriate.  Unfortunately
it's annoying to check whether we have the /right/ scalar field; we
could check, but that does a lot of computation for little benefit.  So
allow any prime-field element in these positions.

I don't (currently) allow a prime-field element as the exponent in `pow'
applied to `MP' or `GF'; I'm open to representation on whether this is a
good idea.  They are permitted in the exponentiation functions attached
to the various modular-reduction classes, because the exponent operand
in those cases is subject to explicit conversion.