Introduce and use ec_encode_scalar()

Introduce and use ec_encode_scalar()

This introduces two "inverses" of the ec_decode_scalar() function that take
a BIGNUM, reduce it modulo p and then encodes it into the curve's field
representation. For setting projective coordinates, we need a specialized
helper that deals with the Z_is_one optimization that is used to optimize
for calculations in standard affine coordinates of the projective plane.
This is used for simplifying EC_POINT_set_Jprojective_coordinates() and
for cleaning up and streamlining EC_GROUP_set_curve().

ok jsing