Add position constraints

[RichardMoot]

Allow specification of constraints of the form X=<Y. These constraints will help with the treatment the reflexives of the Displacement calculus, but they are a simple and powerful mechanism in general.

The constraint-like implementation will essentially be the following
X =< X <=> true
X =< Y, integer(X), integer(Y) <=> Prolog's X =< Y for integers (evaluates to true or false)
X =< Y, Y =<X <=> X = Y
X =< Y, Y =< Z => X =< Z (only rule which adds constraint)