Add assertions

[YichiZhang0613]

The comment requires that the following conditions should hold: inclusive_min < exclusive_max and inclusive_min <= around <= exclusive_max, so corresponding assertions should be added.

	/// the following must hold or else this is liable to panic or produce nonsense values:
	/// - inclusive_min < exclusive_max
	/// - inclusive_min <= around <= exclusive_max
	#[must_use]
	pub fn new(around: usize, inclusive_min: usize, exclusive_max: NonZeroUsize) -> Self {		
		Self {
			around,
			inclusive_min,
			exclusive_max,
			next_change: 0,
			next_op: PlusOrMinus::Minus
		}
	}