Improvement idea - globally configurable helper functions

I was diagnosing performance of an app scanning node_modules. It turned out half of the time is spent in isDirectory on the few cases where it was looking for a missing folder.

What helped was passing in an isDirectory implementation that bails out if the path has gone out of the current folder.

For cases of using `resolve` hundreds of thousands of times in a build process etc. doing a full tree folder listing and caching it would provide a great deal of performance improvement.

For that purpose, setting isDirectory and other helper functions on the `resolve` module singleton instead of drilling them down to where it's called would be useful.

Is that an interesting/welcome contribution?