AoC: property.orderable should provide implementation of type.equality

[fridis]

Found this during soling 23 Dec Advent-of-Code: Since a total order implies an equality relation, property.orderable should implement type.equality as follows

  redef property.orderable.type.equality(a,b property.orderable.this) 
    post
      result = (b <= a <= b)
  =>
    a <= b <= a