Size units (sx, sy)

[Mattriks]

• I've built the docs and confirmed these changes don't cause new errors
• I've added and/or updated the unit tests
• I've run the regression tests

This PR:

• Introduces sx, sy units which are essentially "relative context units" e.g. 20cy + 1sx makes sense
  even when the context origin point is not (0cx, 0cy)
• sx, sy units are user friendly (users don't need to know Compose's resolve functions)
• Fixes ngon, star and xgon (fixes ngon and xgon are transformed incorrectly in UnitBox? #406)
• In the future, other forms in Compose may be simplified by using sx, sy units

Example

In this example, the polygon is sized in both sx and sy units

polyf(x, y, s::Compose.Length) = [(x*cx, y*cy-s), (x*cx+s, y*cy), (x*cx, y*cy+s), (x*cx-s, y*cy), (x*cx, y*cy-s)]

gridf(xs, ys, args...) = (context(),
     line([[(x, 0h), (x, 1h)] for x in xs]), line([[(0w, y), (1w, y)] for y in ys]), args..., linewidth(0.1mm))

using Random
 x, y = [-8; -4:2:4; 8], [-2.5, -2.5, 2.5, -1, 1,-2.5,2]
gx, gy = -10:10, -5:5
rainbow = ["darkviolet","indigo","blue","green","yellow","orange","red"]

img = compose(context(units = UnitBox(-10., -3, 20, 6)), 
    gridf(gx, gy, stroke("gray")),  fill("transparent"),
    (context(), polygon(polyf.(x, y, 0.5sx)), stroke(shuffle(rainbow))),
    (context(), polygon(polyf.(x, y, 0.5sy)), stroke(shuffle(rainbow)))    
)

[Mattriks]

I'll merge this if there are no comments.