Refactor runtime to support AudioPlayer

[JD557]

Refactors the runtime so that the audio player can be used just as easily as the Canvas.

This is a large PR with a ton of changes:

• Low-level subsystems are now abstracted
• Manager classes (e.g. CanvasManager) were removed in favor of plain Function0
• RenderLoop completely refactored:
  • RenderLoop renamed to AppLoop
  • Loop class was removed
  • ImpureRenderLoop/PureRenderLoop distinction was removed. The effect is now picked automatically with a FrameEffect typeclass
  • The impure render frame operation is now State => Subsystem => Unit instead of (Subsystem, State) => Unit
  • Currying changed (see the new AppLoop.Definition)
• MinartApp moved from pure to core, now it can also be used in impure programs
• Added the new AudioPlayerIO operations

I'm not entirely happy with the results (namely, composing subsystems seems unnecessarily complex), but some of the changes seem to be on the right direction for #18