Switch to C89 comments in prep for making the whole codebase C89 compatible.

This should be whitespace/comment only changes and should produce the same
object code.
Correct .gitignore to correctly account for build-aux.
Merge pull request #106

8ca6a9c Correct .gitignore to correctly account for build-aux. (Phillip Mienk)
Merge pull request #105

71712b2 Switch to C89 comments in prep for making the whole codebase C89 compatible. (Gregory Maxwell)
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.