[API CHANGE] Use secp256k1_ec_ prefix for non-ECDSA key operations
Move non-ECDSA operations from ecdsa to eckey
Use internal secp256k1_eckey_ prefix for functions in eckey
Abstract out tweak logic to secp256k1_eckey_* functions
Merge pull request #72

eb74c36 Abstract out tweak logic to secp256k1_eckey_* functions (Pieter Wuille)
ffffc87 Use internal secp256k1_eckey_ prefix for functions in eckey (Pieter Wuille)
e2f71f1 Move non-ECDSA operations from ecdsa to eckey (Pieter Wuille)
ae6bc76 [API CHANGE] Use secp256k1_ec_ prefix for non-ECDSA key operations (Pieter Wuille)