Optimize ScalarMult using endomorphism

This implements a speedup to ScalarMult using the endomorphism available to secp256k1.

Note the constants lambda, beta, a1, b1, a2 and b2 are from here:

https://bitcointalk.org/index.php?topic=3238.0

Preliminary tests indicate a speedup of between 17%-20% (BenchScalarMult).

More speedup can probably be achieved once splitK uses something more like what fieldVal uses. Unfortunately, the prime for this math is the order of G (N), not P.

Note the NAF optimization was specifically not done as that's the purview of another issue.

Changed both ScalarMult and ScalarBaseMult to take advantage of curve.N to reduce k.
This results in a 80% speedup to large values of k for ScalarBaseMult.
Note the new test BenchmarkScalarBaseMultLarge is how that speedup number can
be checked.

This closes #1