Merge #459: Add pubkey prefix constants to include/secp256k1.h

Merge #459: Add pubkey prefix constants to include/secp256k1.h

bc61b91 add pubkey prefix constants to include/secp256k1.h (Andrew Poelstra)

Pull request description:

  In future multisig implementations we will need to pass nonces around, which are algebraically pubkeys but should not be decodable as pubkeys. The way to do this is to change the prefix byte from the ordinary 0x02/0x03 to something else. However, some forks (notably `secp256k1-zkp`) have started using some bytes for their own encodings, and if we continue to use hardcoded constants the risk of conflict is increased.

  This commit puts the prefixes used by the main library into the `include/secp256k1.h` so that the constants we're using will at least be in a standard easy-to-reference place.

Tree-SHA512: 37fa25be5074b7c519a9c69421320a62f32a3818f144254eb57f96c6657b993fc01962a5c670574275d1c59b095a6c89e60736123f032d6736907284eac526d7