musig-spec: address robot-dreams' comments
musig-spec: address robot-dreams' comments

- KeyAggCoeff' -> KeyAggCoeffInternal for consistency
- In Sign, add mod n when calculating d
- In Tweak, reorder the parameters to (Q, gacc, tacc, tweak, is_xonly) because
  the first three are "state" arguments
- Rename Tweak function to ApplyTweak to avoid confusion with tweak (the
  vector). This becomes apparent in the python reference code.

ElementsProject/secp256k1-zkp,ElementsProject/secp256k1-zkp