Add bench_sign tool
Add verification to 32bit field

- implement _fe_verify for 10x26
- fe is normalized after _fe_clear
- a few corresponding changes in 64bit field
Better .gitignore for bench binaries
Merge pull request #83

7d681ac Add verification to 32bit field (Peter Dettman)
Merge pull request #82

8f9a307 Better .gitignore for bench binaries (Pieter Wuille)
fa5c13f Add bench_sign tool (Pieter Wuille)
Avoid division when decomposing scalars

- In secp256k1_gej_split_exp, there are two divisions used. Since the denominator is a constant known at compile-time, each can be replaced by a multiplication followed by a right-shift (and rounding).
- Add the constants g1, g2 for this purpose and rewrite secp256k1_gej_split_exp accordingly.
- Remove secp256k1_num_div since no longer used