Improve normalization performance for 32bit

- Uses a similar approach to the latest 64bit _normalize.
- Add one useful optimization back into the 64bit _normalize too.

Performance of 'bench' improved by around 0.5% for the 32bit field (but tested on a 64-bit machine).
Merge pull request #40

42822ba Improve normalization performance for 32bit (Peter Dettman)
Use Co-Z arithmetic for WINDOW_A 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 point precomputations in secp256k1_ecmult.
- WINDOW_A size increased to 6 since the precomputation is much faster per-point.
- DBLU cost: 3M+4S, ZADDU cost: 5M+2S.
- From 2.4% to 3.8% faster 'bench' results, depending on configuration.
Merge branch 'coz-arithmetic'