New iterator factory methods to support Fortran looping

Added

• Added Fortran-friendly iterator factories (and tests) to

  ftn_begin() and ftn_end() have been added to vector, set, map,
  and ordered_map templates. ftn_begin() points to just-before the
  1st element of a container, while ftn_end() points to the last
  element. This allows the next() invocation to be at the start of the loop which
  is less error prone in the presence of CYCLE statements. Example usage:

  type(Vector) :: v
  type(VectorIterator) :: iter
  ...
  associate (e => v%ftn_end())
     iter = v%ftn_begin()
     do while (iter /= e)
        call iter%next()
        ...
        if (<cond>) cycle ! does the right thing
        ...
     end do
  end associate