a hint more details when trying to create a directory over a file
Fix the problem of copying a directory onto its parent (and having the destination name inferred).

The logic for "is a parent of" has been moved into SwiftObjectPath.isEqualToOrParentOf()
clean up & so stop stream related warnings
move TestSwiftFileSystemBasicOps under SwiftFileSystemBaseTest.

This gives better startup and teardown, lets us pull duplicate (static) methods.

As part of this, all test files are under /test/
Improve and reformat documentation.