[API BREAK] Use a nonce-generation function instead of a nonce
Implement SHA256 / HMAC-SHA256 / RFC6979.
Use rfc6979 as default nonce generation function
Merge pull request #163

bbd5ba7 Use rfc6979 as default nonce generation function (Pieter Wuille)
b37fbc2 Implement SHA256 / HMAC-SHA256 / RFC6979. (Pieter Wuille)
c6e7f4e [API BREAK] Use a nonce-generation function instead of a nonce (Pieter Wuille)
Use Co-Z arithmetic for precomputations

- Selected Co-Z formulas from "Scalar Multiplication on Weierstraß Elliptic Curves from Co-Z Arithmetic" (Goundar, Joye, et. al.) added as group methods with new type sep256k1_coz_t.
- Co-Z methods used for A and G point precomputations.
- WINDOW_A size increased to 6 since the precomputation is much faster per-point.
- DBLU cost: 3M+4S, ZADDU cost: 5M+2S.
- Take advantage of z-ratios from Co-Z to speed up table inversion.
Demonstrate "split-Z" secp256k1_ecmult