[m-rev.] Handle polymorphic inequality goals

[Ralph Becket]

After discussion with Fergus, we've come to the following decision:

- Keep `<', `=<' etc. as module definable operators for int, float etc.
  with the appropiate semantics (IEEE or whatever.)

- Move the definitions of `@<' etc. from prolog.m to builtin.m with
  the understanding that X @< Y <=> compare((<), X, Y) etc.

- Resurrect Simon's change to support types with user-defined compare/3
  behaviour.

- It's not clear that adding an (in, in, in) is semidet mode to
  compare/3 will gain us very much under these circumstances.

- Add a function

  	:- func ordering(T, T) = comparison_result.

	ordering(X, Y) = R :- compare(R, X, Y).

- Ralph (not a bullet point.)
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