This commit adds proving and verification functions for discrete
logarithm equality.
From the spec (https://github.com/discreetlogcontracts/dlcspecs/pull/114):
"As part of the ECDSA adaptor signature a proof of discrete logarithm
equality must be provided. This is a proof that the discrete logarithm of
some X to the standard base G is the same as the discrete logarithm of
some Z to the base Y. This proof can be constructed by using equality
composition on two Sigma protocols proving knowledge of the discrete
logarithm between both pairs of points. In other words the prover proves
knowledge of a such that X = a * G and b such that Z = b * Y and that
a = b. We make the resulting Sigma protocol non-interactive by applying
the Fiat-Shamir transformation with SHA256 as the challenge hash."
