Common code for other FieldElement implementations
Moved I to Curve, pass it in as a parameter

Most FieldElement implementations (in particular, the ref10 implementation)
don't have a generic pow() method, and instead will have I hard-coded.
Radix 2^51 implementation of FieldElement from ref10

Before black magic:
0.5548 each sign, 2.4118 each verify, 2.9666 s+v
After black magic:
0.1572 each sign, 0.2798 each verify, 0.437 s+v