Lots of improvements to the path flow.

- Interpolation is now done at the motion level
  as opposed to the higher path lever
  so an arcCW when not supported by the driver
  can be interpolated into line segments.

- Arcs are detected in offsetting and their radii
  is offset so they can be preserved all the way
  to the driver in offsetted fills and strokes.

- Cleaned up the way helical plunging tracks distance.
Make depth savable
Add fillCircle() convenience method.
Fix issues with ctx.top

- Remove ctx.aboveTop. It was never really necessary. Everything can be
  done simpler by using ctx.top.

- Settled on a good lathe API.
Remove superfluous stuff from readme
update readme
Optimize inner strokes to climb mill.
Improve fill strategy to be more tolerant to non-center cutting end mills
Simplify text implementation
Lathe work
Implement arcTo finally
update readme
Add ctx.precision for controlling number of divisions
Add example screenshot
Add better example to readme
Remove stroke simplification that was accidentally added
Update readme
Add real life example
Up default precision
Add back mod pi for arcs