Make module transformer the last transformer

[arv]

This is so that other transformers can output import declarations in the future.

There are two real changes in here, both related to export declarations:

ClassTransformer: Make sure that export default class X {} generates valid code.

DestructuringTransformer: Make sure we do not export the temp variable.

[arv]

@johnjbarton PTAL

[johnjbarton]

LGTM

[arv]

I'll also add a feature test that ensures that a module instance object does not have any extra properties. Before this PR a module instance object would have a $__0 property if someone exported using destructuring in an export declaration.