Signature's S, when over half order, has to be encoded differently

Reference implementation: https://github.com/bitcoin/bitcoin/blob/b7bba43a14a641b6b64bbf48357b2987c1b05800/src/key.cpp#L235-L238

ht @oleganza

Thoughts?
I went through all of the call sites across all Conformal code and making this change without a flag is not a problem since nothing outside of this test relies on being able to exactly reproduce old signatures without the new low S functionality.