Export from bundles at assignment time, rather than with a final getter

This fixes #17, wherein chained exports from bundles (e.g. `import { foo } from 'bar'; export { foo };` in the entry module) were transformed incorrectly.

Rather than using a somewhat inelegant method involving `Object.defineProperty()`, exports are now exported at assignment time, rather than en masse at the end. This is the same as happens with one-to-one transformations.