Allow for ordinary (non xonly) tweaking of MuSig keys

[nickfarrow]

tweak(tweak, is_xonly) now takes a bool as to whether the new tweak should update the accumulated needs negation. Also adds the remaining tests from the draft BIP.

• If the tweak is not x-only (an ordinary tweak), then never negate the aggregate key prior to applying the tweak, nor negate the existing tweak. Also do not update the MuSig keylist's accum_needs_negation.

• If the tweak is x-only, then do the negation if required as usual.

I changed the name to accum_needs_negation so we can tell the code is using this spec. Not yet added to upcoming FROST PR.

I think we should review the updated logic together before merging.

see https://github.com/jonasnick/bips/blob/musig2/bip-musig2.mediawiki#Tweaking_Definition https://github.com/jonasnick/bips/issues/13

[nickfarrow]

ah I had the title right the first time. Ordinary tweaks are for BIP32, is_xonly=false,