Made CreateActions start using the Concrete FS

This required some minor changes to the concrete FS, on account of the
requirements of the File and Dir create actions. Primarily, there was a change
to accept the type of a lookup, rather than a suggested constructor. This is
backwards compatible with the existing usage so far (less an argument name
change), but importantly allows Concrete/Virtual FS agnostic code that does
lookup(path, elem_type=abstract.File) in order to avoid having to choose
Concrete or Virtual File types at the location in which the lookup occurs.