beziers now support dashed-line strokes

- added `dash` arg to pen()
- separated plotstyle inheritance into ColorMixin and PenMixin
beginnings of plot() command

- should eventually be able to draw any grob, apply inline
modifications, and detect whether it’s already part of the canvas (in
which case use a copy).
expand every tilde
out with the old
rect() now takes a `radius` arg to set corner roundness

- this triggers nsbezierpath’s handling of roundrects (and can take an
(xradius,yradius) pair)
- specifying the `roundness` arg instead uses the nodebox behavior of
manually drawing a rounded path
syntax updates
error-checking errors