weak normalization
Weak normalization for secp256k1_fe_equal
Add _fe_normalizes_to_zero method
Add _normalizes_to_zero_var variant
Merge pull request #154

49ee0db Add _normalizes_to_zero_var variant (Peter Dettman)
eed599d Add _fe_normalizes_to_zero method (Peter Dettman)
d7174ed Weak normalization for secp256k1_fe_equal (Pieter Wuille)
0295f0a weak normalization (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.