Precision Adjustable Z Endstop Holder and Constraint (Revisited)

A precision adjustable optical endstop holder for Prusa printers, with an integrated Z screw constraint bearing.

I was finding that reaming out the slide hole could be quite a challenge, so I've incorporated triffid_hunter's bushing, to make things easier.

I also added a matching Z screw constraint clamp for use on the other side of the printer.