Allow caller to optimize re-runs

[natevw]

Right now the function wrapper has to crawl through the window prototype chain every time it is called. And once we integrate the iframe trick in #1 that setup would add additional overhead.

In some cases with repeated calls to wrapped functions where decent performance is desirable (e.g. calling a PouchDB map function over a whole bunch of documents) the caller may know that globals haven't been added. We could add a .evel_reuseSandbox property on the wrapped function to speed up repeated calls.