Functions to inverse and negate private key, commit with 2 blinding factors

[jaspervdm]

This PR adds 3 new convenience functions (and corresponding tests):

• secp256k1_ec_privkey_tweak_inv - inverts the private key (x 🡒 x^(-1))
• secp256k1_ec_privkey_tweak_neg - negates the private key (x 🡒 -x)
• secp256k1_pedersen_blind_commit - pedersen commitment with 2 blinding factors instead of blinding factor and u64

[garyyu]

And you also add secp256k1_pedersen_blind_commit function in this PR.
What's it for?

[jaspervdm]

Yeah haven't updated the PR description yet, will do soon. It is to calculate commitment x*G + v*H where v is a secret key instead of a 64 bit integer

[jaspervdm]

I added some tests for the secp256k1_pedersen_blind_commit function, the PR is ready for final review!

[ignopeverell]

Could you provide some general direction as to what those could be used for?

[antiochp]

@jaspervdm doing some secret dev work by the looks of it.

[jaspervdm]

These 3 methods are useful in a secret sharing scheme. In particular, the inverse and negate functions are used in calculations for Lagrange basis polynomials.