Change code gen for module getters

This changes the code gen for modules to return a simpler object literal. If someone asks for a module instance object, using `module ident from 'name'` or `System.get('name')`, we create the `Module` object at that point.

The benefit for this is that we now have a smaller code gen and it allows follow up changes to dynamically do `export * from 'name'`.

Partial fix for #470

Review URL: https://github.com/google/traceur-compiler/pull/474

Closes #474.
Dynamic export star

Introduce a runtime function that handles `export *`.

This allows us to not need the module symbol at all in the transformation phase.

Fixes #470

Review URL: https://github.com/google/traceur-compiler/pull/475

Closes #475, closes #470.