Use vfs.FileSystem instead of own interface.

Closes #11.

Getting the ball rolling on this one. It seems to be okay so far. I had to update `vcsstore` accordingly and will make a separate PR for that. It should work in the interim due to vendoring.

I didn't test the native path, only the cmd-based stuff (via [this](https://github.com/shurcooL/play/blob/master/48/main.go) and [this](https://github.com/shurcooL/play/blob/master/63/main.go) compiling and working) so far.

Just wanted to get this in front of your eyes so you can look it over and give feedback.
This is no longer needed since one can use `mapfs.New()` directly.