Parametric standoff/spacer generator

I needed some plastic spacers to finish off assembly of a Pi case and needed the kind of spacer into which the screw can thread itself. Found these, but unfortunately they were the “pass through” kind … so I decided to make my own spacers, in open OpenSCAD format.

It's one simple module with pre-defined parameters which you can override:

* `spacer_type` is the “roundness” of the spacer; setting it to 6 gives you a nice hex, more than 120 gives you a nicely shaped cylinder
* `spacer_height` is how tall the spacer is
* `screw_diameter` is the diameter of the screw; the inner hole size is adjusted using the `plastic_coefficient` parameter, depending on the material you're printing with (the rigid plastic works OK for ABS/ASA/PETG too, but it will be much more difficult to thread the screw into the plastic; however, the spacer will be reusable, unlike the “loose” ones which are pretty much single use)
* `screw_length` is the length of the screw; making `spacer_height` equal to 2*s