Improve normalization performance for 32bit

- Uses a similar approach to the latest 64bit _normalize.
- Add one useful optimization back into the 64bit _normalize too.

Performance of 'bench' improved by around 0.5% for the 32bit field (but tested on a 64-bit machine).
Merge pull request #40

42822ba Improve normalization performance for 32bit (Peter Dettman)
Merge branch 'master' of https://github.com/bitcoin/secp256k1 into compact-mul