If an iterator facade (like entity iterators) wants to allow the
embedded implementation to return either an (internally stored)
reference or a temporary object and expose these two behaviors to enable
performance optimizations, operator->() needs special handling: If the
implementation returns a reference, operator->() in the facade can
simply return the address of the referenced object, but if the returned
object is a temporary, we need to capture and store it in a helper
object to make sure it outlives the member access.

This patch adds a little helper function that tansparently handles both
cases.