mesh_t based ruby bindings

This PR will update the ruby bindings for the `csg` gem to be based on the new `mesh_t` type from #49 

## Tasks

  * [ ] Wrap `mesh_t` into `CSG::Native`
  * [ ] Wrap `mesh_read_file` to return a `CSG::Native`
  * [ ] Update `CSG::Solid` to be `mesh_t` backed instead of `stl_object`
  * [ ] Map appropriate methods for I/O and operations

## Maybe this also?

  * [ ] Remove the `stl_object` from being exported to ruby?