Range: pure whitespace fix
Range: make now returns Or_error and clean up

Also remove total orders, stop using Order module, and remove to_pair
function. None are useful enough to justify adding to the API.
RSet.to_canonical: stop using Order module

It is more clear to define the order relation directly than to use an
extra abstraction.
Internal_pervasives: delete Order module

It is no longer used and not useful.