don't trigger generators with stat() or filler dir's readdir()

[matthewmueller]

This PR reduces the scope of the virtual filesystem in order to reduce the number of calls to the generator functions.

After this PR:

• stat.Size() will always return -1 for generators
• stat.ModTime().IsZero() will always be true for generators
• fs.Stat(genfs, filepath) won't call the underlying generate function
• fs.ReadDir(genfs, somedir) won't call the underlying generate function of the dir entries

This is a tradeoff, but I think it's going to be worth it. It will be more clear when generators run at the expense of being a less accurate filesystem. Surprisingly only a few of the tests failed and none of the framework tests failed after this change.

[matthewmueller]

Closing in favor of #268.