Generic Mesh Type

This work adds `mesh_t` and assorted methods for having a single type to deal with for CSG interactions. This work is the foundation for additional I/O formats.

### Plan

  * [x] `mesh_t` base and methods 
  * [x] `stl_mesh_t` prototype
  * [ ] Input should produce a `mesh_t` type instead of `stl_object`
  * [ ] Use `mesh_t` methods to produce BSP trees instead of `stl_to_*` and `*_to_stl` methods.