Add speedup for sig verification based on secp256k1.
Some optimizations for Hal's secp256k1 verifier
Precalculate P, N and G*lambda in CSecp256k1Consts
Refactor secp256k1 ECDSA verify code to mimic generic version
Allow verification of optimized secp256k1 code
Implement Hal's suggestion for a faster G split
Build test objects separately