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)
Merge branch 'master' of https://github.com/bitcoin/secp256k1 into coz-arithmetic