Split System.register into .register() and .registerModule().

'instantiate' -> .register()
'register' -> .registerModule()
This allows the two formats to evolve independently.

Add `require` argument in prep for sending relativeRequire via function arguments.
If I'm building a non Node.js app I don't really want this extra overhead.

I don't really have a suggestion. Dead code removal should remove it from the web facing apps.