RandomAccessIterator requirements include LessThanComparable, so
implement the missing comparison operators for this class, as well as
for const_reverse_iterator.
This also fixes compilation problems with MSVS 2013 in debug mode, where
the CRT uses these operators to check the iterators correctness.
See https://github.com/wxWidgets/wxWidgets/pull/1048
aee926f2d5
