RemoveAll & MkdirAll to Filesystem interface

[lordofscripts]

For some reason the original author chose to implement the equivalent of the underlying OS os.RemoveAll() and os.MkdirAll() as global vfs functions and left them out of the vfs.Filesystem interface.

• Move RemoveAll & MkdirAll to Filesystem interface

[lordofscripts]

As a transition, the memfs & bucketfs MkdirAll and RemoveAll method implementations would actually be an alias to their respective vfs functions. For now that seems pretty sensible without code duplication.