Introduce secp256k1_scalar_t for future constant-time mod order operations
Merge pull request #75

a9f5c8b Introduce secp256k1_scalar_t for future constant-time mod order operations (Pieter Wuille)
Fix build for 64bit field under OSX

- caused by https://github.com/bitcoin/secp256k1/commit/8881212ebc43e67052ec06dec8beb459769fbab7
- OSX's ar tool doesn't work for empty archives ("ar: no archive members specified")
- repaired by moving entire commit under USE_ASM