Add low-level support for hatching closed polygon. This supports polygons with holes via compound Path. In such case, the first sub-path is considered to be the exterior boundary, and the subsequent sub-paths holes.

Change summary:

• added low-level hatch_polygon and HatchParams
• added Polyline::hatch(), FlattenedPath::hatch() and Path::hatch()

This is part 3 of 6 in a stack made with GitButler:

•  6  Add support for shape filling to whiskers #188
•  5  Introduce IntoBezPaths trait #191
•  4  Add support for layer/path metadata inheritance #187
•  3  Add hatching algorithm to Path and FlattenedPath #186 👈
•  2  Add .buffer() to Path and FlattenedPath #184
•  1  Add low-level implementation for path joining and merging optimization #183