Don't save setter for const exports.

[John-David Dalton]

This would address a line item on #90. It would also be a good time to think about the parser optimization to do a top-level only parse if there are only const exports and nested imports is disabled.

[Ben Newman]

Specifically, we can discard the setter after the first time it is called, if module.loaded is true, meaning we should have finished initializing all top-level const values.