Added additional constructors and comparison operators

This adds more converting constructors for easier conversion between
map::Reference and simple::Reference.
Also adds comparison operators to all api::Reference implementations and
simple::Key.

Change-Id: I22661180f7ed8f640edb162ff4fc7b3a1ccd53cd
Signed-off-by: Thomas Psota <thomas.psota@iese.fraunhofer.de>
13 files changed