shapeII: rewrite

- handle the case when 4th arg is missing
- pass all information to subroutines as parameters (except slur-dir)
- add relative-polar mode
- don't use let* when not necessary
- renaming
- remove some set! usages
shapeII: support accumulation
shapeII: add basic notehead positioning
shapeII: support cross-staff notehead positioning
remove shape-polar - it's now fully merged into \shapeII
shapeII: make 'side' use LEFT/RIGHT
shapeII: example, README update
shapeII: change order of polar coords to reflect math convention
shapeII: change polar mode names and add another one
shaping-curves: clean the directory, move examples around
shaping curves: rewrite examples
shapeII: direction fix in 'head' instruction, swap args (to be more intuitive)
shaping curves: update README