Makerfarm Prusa i3 Small Bed Parts replacements

NOTE: this isn't a perfect design. It required a significant amount of cleanup afterwards, due to the tolerances on the linear bearing holders not being correct.
But hey, the openSCAD file isn't hard to edit.

So these are some improved parts for the Makerfarm Prusa i3 8" (linear bearings, not the v-slot stuff)

The y-idler piece has improved nut holders, which make it less annoying to assemble.
The y-carriage belt holder now is a single piece, with a nut trap so that it can be tightene