Enforce low S values, by negating the value (modulo the order) if above order/2.

Reference implementation: https://github.com/bitcoin/bitcoin/blob/b7bba43a14a641b6b64bbf48357b2987c1b05800/src/key.cpp#L235-L238

ht @oleganza
IMPORT PATH MODIFICATION CHANGESET
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Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.