Fix code comment inconsistency and add corresponding tests

[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
		}
	}

I also add corresponding tests.

[YichiZhang0613]

@itsjunetime Hi, sorry for the interruption. Could you please review this PR when you have time? It's been open for two weeks and I'd appreciate any feedback. Thank you very much!