Restructure Engine Namespaces / Areas

[capnkenny]

From #422 -

Post MVP I want to restructure the engine such that each "area" is its own static/object library, and is then linked into the main "engine" - this should simplify the layout of our include files and flatten them so that we can use PUBLIC_HEADER or whatever. Hopefully this'll enforce a lot more separation between various components.

Issue opened for tracking and discussion