[WIP] Improvements to the kernel, including an example.

This is the simplest way I could find to clean up the memory allocated for meshes rendered in Libfive. There might be a way to add a destructor directly to subclasses of Structure like MeshStruct, but I'm not good enough at Python to figure out if this can be done. This merge request does eliminate the memory leak that I was aware of.

I also added an example to demonstrate rendering a shape to a mesh and displaying it in PyInventor.