Wingman case split does the wrong thing on nested binds

[isovector]

@TOTBWF reports:

Found a case splitting bug

proofs :: forall ext err s m goal. (MonadExtract ext m) => s -> ProofStateT ext ext err s m goal -> m [Either err (ext, s, [goal])]
proofs s p = go s [] pure p
    where
      go :: s -> [goal] -> (goal' -> ProofStateT ext ext err s m goal) -> ProofStateT ext ext err s m goal' -> m [Either err (ext, s, [goal])]
      go s goals cont p1  = _

splitting on p1 causes this

proofs :: forall ext err s m goal. (MonadExtract ext m) => s -> ProofStateT ext ext err s m goal -> m [Either err (ext, s, [goal])]
proofs s p = _
proofs s p = _
proofs s p = _
proofs s p = _
proofs s p = _
proofs s p = _
proofs s p = _
proofs s p = _
proofs s p = _

[isovector]

This is caused by the same underlying issue as #1516 --- namely I'm grafting decls when I should be grafting matches.