### 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.
Reasoning in comments. The main motivation is to avoid all the case
checks when walking the filesystem.
"again" as we already tried once, but it caused a major issue ragarding
mtimefs layer. The root of this problem has been fixed in the meantime
in ac8b3342a
* lib/fs, lib/model: Add error channel to Watch to avoid panics (fixes#5697)
* forgot unsupported watch
* and more non(-standard)-unixy fixes
* and windows test
* review