Ender 5 ModMax v02 - upgrade with CoreXY and 310x310 bed

This is still in development, but some people asked about it. Updates will follow as I work towards a final version.

This is a refined and optimized version of my previous Ender 5 ModMax design. I was looking to improve three aspects of the design:

1. Better accommodate the 310x315mm print bed from the CR-10. It was a very tight fit on the previous design and this design has much improved Y and slightly better X to make full use of the print bed.
2. Reduce material use and design complexity. I redesigned the parts from scratch, making them as simple as possible and using a minimum of material while maintaining excellent strength and ease of printing.
3. Move away from the Stealthburner mount. Microcenter sold a bunch of Creality parts for next to nothing back in the Fall. I scored several Sprite Pro extruder / hot-ends setups for $15-20 each, so I wanted to use them. I couldn't find mounts that worked with them, so I made my own modular and easily swappable mounting system. 

Some of the significant changes are:

• Reduced part size and design complexity – the best design is simple, elegant, and uses only the minimum amount of material.  As a result, this design is not backwards compatible with the ZeroG Mercury One design. 
• XY steppers are mounted from the bottom to save space and for added strength.
• Switched to MGN9 rails (space and cost savings).
• Custom hot-end universal mount that slides together and is held in place by a single screw. Inspired by the one designed by Proper Printing.  This make it very easy to accommodate new hot-ends and is more space efficient in the Y dimension than the Voron Stealthburner mount that I initially used. 
• Hard mounted print bed with dual Z-axis that reuses parts from the Ender 5. I found using magnets and sliding mechanisms did not improve print quality and actually made it worse in many cases. The bed is rarely more than a millimeter out of level, so the ability for each corner to be raised/lowered independently is overkill and unneeded. This is why newer printers from Bambu Labs, Creality, and Prusa don't use it. Once it is leveled, it doesn't move, and what little movement there is can be compensated by the heightmap.  

Refinements to be added:

• Integration of Y limit switch into the stepper mount.
• Improved cable chain attachment point and X limit switch on the hot-end mount. It will stay modular, but the current design is merely functional with lots of room for improvement. 

Please let me know if you have any suggestions or feedback.