Consider nfield instead of big.Int for a speedup

[bmperrea]

as in here btcsuite/btcd#621

Check some benchmarks for speedup.

[zquestz]

@bmperrea Yep this is on my list to check out. I am walking through all the PRs and pulling in all the good stuff. Thanks for the heads up!

[zquestz]

@bmperrea if I were to finish up Jimmy's PR and get everything building, would you be willing to help me solve the remaining constant time issues?

[bmperrea]

definitely interested to help with the constant time stuff 👍 . I do think calling out to a C lib like sekp256k1 is a bit safer than rolling our own implementation of the crypto here, but it's interesting stuff :)