Compact Geared Bowden Extruder

This is a combination of parts that adds up to a fairly compact geared extruder, with mounting ears to attach to 2020 extrusions. See HC300-NOTES.md for assembly instructions.
exploded_view.stl shows how everything goes together.

I haven't yet gotten around to finishing my Hypercube, but I recently converted my A8 to bowden extrusion, so I whipped up a mounting adapter and put it on the A8 for testing. Preliminary testing looks good so far.

(Project mirrored at https://gitlab.com/salf