Implement internal math module

[samuelselleck]

This implements an internal math module, to be exposed to users of pax, and removes the use of kurbo::Affine as the engines main transform type.

[zackbrown]

Nice improvements! Small changes requested re: design_utils, per team conversation. After that, please go ahead and merge directly.

[samuelselleck]

This is done now!

• Created a new NodeInterface type that is a public wrapper over ExpandedNode
• Added methods on NodeContext to do raycasting, finding by id, etc (forwarding calls to render_context and wrapping results in NodeInterface)
• Renamed NodeContext to EngineContext to better reflect it's ability to do things like global raycasting

[warfaj]

LGTM! Thanks for the changes.