Serious business Bear MK2, MK2.5, MK3 x-axis replacement

TODOs
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- Finish writing instructions.
- Finish writing bill of materials.
- Orient STLs more reasonably.
- Take some pictures.

Background
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I wanted a stiffer, more-awesome X axis for my Bear printer with a more-durable idler. Like the Serious Business y-axis replacement, this one uses external bearings in the idler. Also like the y-axis it uses press-fit linear bearings to maintain stiffness and straightness.

This mod is somewhat easier and less invasive than the "fu