Wrapper lazy instantiation

[pmdartus]

When playing around with #155 I came to the realization that for each created impl, webidl2js eagerly creates the associated wrapper. However, 99% of the jsdom code operates on the impls not the wrappers, window.js is the only exception. Those wrappers are only needed when userland code manipulates the DOM APIs.

I am wondering if it could be possible to make all the wrapper instantiation lazy to reduce the overall jsdom memory footprint.

[ExE-Boss]

That would probably also make #155 not cause a 3× slowdown on V8.