The standard type imposes some additional requirements that are not
necessary here, but using this concept allows standard containers to be
classified correctly without specific overrides.
Prior to this, an xrange always started at the begin term and
incremented by 1 per step until it reached the end term. There was no
support for a step size other than 1. Add support for custom step size.
It is the caller's responsibility to pick a step size that will
eventually lead to (iter != end) evaluating to false.
