### Purpose
The `fs.Filesystem` interface contains two parts that cannot be
implemented externally because they are private:
* `filesystemWrapperType`: this PR changes `unwrapFilesystem` to
downcast to a specific concrete type
* `underlying`: this PR simply moves it to an unexported interface
### Testing
Regular tests pass.
I don't really understand under what circumstances, but sometimes these
calls panic with a "panic: counter cannot decrease in value" because the
value passed to Add() was negative.
Jakob Borg
Tommy van der Vorst