Merge pull request #540 in G/fastr from ~ZBYNEK.SLAJCHRT_ORACLE.COM/fastr:refCountFix to master

* commit '93d25a35':
  EmptyTypeSystemFlatLayout annotation added to the attribute nodes.
  UpdateShareableChildValueNode moved to com.oracle.truffle.r.nodes.function.opt
  Package documentation for the new attributes model
  Iterative attribute nodes never return null
  A better profiling in the ref counting nodes
  A fix-up of refence counting when setting an attribute