Remove contrib/intarray's definitions of the <@ and @> operators, so that they

don't cause confusion with the built-in anyarray versions of those operators.
Adjust the module's index opclasses to support the built-in operators in place
of the private ones.

The private implementations are still available under their historical
names @ and ~, so no functionality is lost.  Some quick testing suggests
that they offer no real benefit over the core operators, however.

Per a complaint from Rusty Conover.