ENH: consistent reverse iterator definitions for UList and FixedList

- consistent with C++ STL conventions, the reverse iterators should
  use operator++ to transit the list from rbegin() to rend().

  The previous implementation used raw pointers, which meant that they
  had the opposite behaviour: operator-- to transit from rbegin() to
  rend().

  The updated version only has operator++ defined, thus the compiler
  should catch any possible instances where people were using the old
  (incorrect) versions.

- updated forAllReverseIters() and forAllConstReverseIters() macros to
  be consistent with new implementation and with C++ STL conventions.