Prusa I3 Hephestos 8mm Z Axis

This is a simple redesign of the original Hephestos Z Axis, in particular the two X axis parts.

- The parts now support a 8mm threaded rod, including a fitting for the nut.
- On the inside there was a small lip added on the bottom preventing the bearings from falling out over time.

Everything else is default for the Hephestos.