Renaming unit test directory
Adding test suite for system tests

System schmystem. At least more end-to-end and just to keep the
contractual end-user usage tests separate from the direct testing of
single methods.
Starting to use a separate superclass for Fiber and DataType

Don't really want Fiber to extend DataType, as they have too many
differences and are not really the same. But they still have a few
things in common, hence the abstract class Combinable.