Use internal secp256k1_eckey_ prefix for functions in eckey
Abstract out tweak logic to secp256k1_eckey_* functions
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