sinsemilla::note_commit: Improve NoteCommit gate layout.

By rearranging the pieces in the gate, we remove a prev() query and
preserve proximity between pieces involved in the same constraint.

This commit also includes several minor fixes:
- use strict mode for decomposition of j in y-coordinate check;
- Name All Polynomial Constraints;
- remove point_repr() helper function;
- variable renaming and docfixes.

Co-authored-by: Jack Grigg <jack@electriccoin.co>