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.