Square of the distance with other on the X and Y axes

Property definitions

geometry $ IPoint :: dist2_xy
	# Square of the distance with `other` on the X and Y axes
	private fun dist2_xy(other: IPoint[N]): N
	do
		var dx = other.x.sub(x)
		var dy = other.y.sub(y)
		var s = (dx.mul(dx)).add(dy.mul(dy))
		return x.value_of(s)
	end
lib/geometry/points_and_lines.nit:86,2--93,4