Rework group tests
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
Update group elements tests with Co-Z and addition with z ratio
Make ge_set_gej_zinv verify its precondition
Add formulas to secp256k1_coz_dblu_impl_var
Split off Z factor addition and change explanation
Remove unused function secp256k1_ecmult_table_precomp_gej_var
Update comments for EC table precomp functions