Switch to Cura because MiracleGrue is abandoned

This is a WIP PR switching to CuraEngine, the slicer used in Ultimaker's Cura slicer.

### TODO
* [x] Remove grue from `vendor/` & replace with Cura Engine
* [x] Compile Cura with `vendor/Makefile`
* [ ] Switch slicing to Cura
* [ ] Ensure it works on the Replicator 2 at GitHub HQ 3.0