Add a simple Filesys object

Defines very few operations, mostly it's able to lookup existing
FilesysElements (and manufacture them when needed), and copy files/dirs/links
from one location to another.
Renaming and cleanup to abstract filesys
Add initial implementation of a ConcreteFilesys

This is a Filesys which passes transparently through to the real, underlying
filesys. Should be a drop-in replacement for the calls to shutil and os
throughout the code (but probably won't be).