AR Grip

Taken from the FossCAD megapack V4.8. AR-15_FOSSCAD_MOE_Grip_v1.1-FP.

Fixed a few things from the original model (fit on receiver was too small, safety selector spring well deepened to proper depth) + added stippling/knurling.

***NO supports needed! I added the thin support for the beavertail so you can save a fairely significant amount of suport material and just snip it off after printing.