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?
Aye.  I just want to double check that nothing in the code currently relies on being able to create these old signature.  I don't believe it does, but I don't take breaking existing tests lightly.