Small API clean up.
Haven't really done much but cleaned up the API a bit. This version sees the removal of the Nu.init () function from the public API since it's now called by the engine automatically.
Also, in an effort to support the best value semantics for Nu's World type, there is now a World.empty value that can be built upon. Not a useful detail for most users, but helps to keep the engine's design on a lofty track.
Gave more value-style semantics to World. Removed Nu.init () BREAKING CHANGE 0d9a936
More work on making World a value. b2dcda9