Add basic pyo3 module functionality / wrappers

[tychedelia]

Changes:

• Adds some more scaffolding around setting up the implicit graphics context for a sketch.
• A simple python sketch!

You can see the first commit for an attempt in which we just store the graphics in a TLS just like we do the Bevy App in processing_render. But then I thought, surely there's a way to stash it in the Python runtime itself, right? There is! If you add pass_module to all the wrappers, you get access to the module object itself which is effectively just a GIL controlled map.

I did a bit of research into prior art and this is also what they are doing, i.e. module level _graphics member. The user can theoretically access this if they want, but then all our module fns just forward to that object.

TODO:

• Start the sketch automatically when loading the module / runtime
• Add more lifecycle functions
• Add Surface class post Refactor rendering crate to clarify primary Processing API #34
• Some image stuff
• Make API pretty (think about colors!)

[catilac]

Yay! I was still trying to figure out more of the details about pyo3, and was about to implement impl From<ProcessingError> which is obviously the wrong direction. I like how you put the Entity and GlfwContext into a class. Thats what I was missing... Thanks!