reverse Function

   elemental function reverse(self) result(reversed)
   !< Return a reversed string.
   !<
   !<```fortran
   !< type(string) :: astring
   !< logical      :: test_passed(2)
   !< astring = 'abcdefghilmnopqrstuvz'
   !< test_passed(1) = (astring%reverse()//''=='zvutsrqponmlihgfedcba')
   !< astring = '0123456789'
   !< test_passed(2) = (astring%reverse()//''=='9876543210')
   !< print '(L1)', all(test_passed)
   !<```
   !=> T <<<
   class(string), intent(in) :: self     !< The string.
   type(string)              :: reversed !< The reversed string.
   integer                   :: length   !< Length of the string.
   integer                   :: c        !< Counter.

   if (allocated(self%raw)) then
      reversed = self
      length = len(self%raw)
      do c=1, length
         reversed%raw(c:c) = self%raw(length-c+1:length-c+1)
      enddo
   endif
   endfunction reverse