add ability to get public key from a pedersen commitment

[garyyu]

To complete the TODO item in repo rust-secp256k1-zkp: need an API in secp to convert commitments to public keys safely. a commitment is prefixed 08/09 and public keys are prefixed 02/03.

A new function secp256k1_pedersen_commitment_to_pubkey is added.

[yeastplume]

I've merged this but just a couple of comments for next time:

• There should be a corresponding definition in the appropriate header in secp256k1/include/
• just as a precaution I'd usually include tests for a new function as well (in tests_impl.h).

I'll add this in myself for now, so just have a look at the next couple of commits for next time.

Thanks again!

[antiochp]

🚀

[garyyu]

Noted with thanks.