Avoid overflow in operations due to +1 or -1.

[ptal]

For example in Difference:

let left = self.intersection(&Interval::min_lb(other.lower() - Bound::one()));

It can overflow if the lower bound is unsigned and equals 0, it is avoidable.

[George-Ogden]

Related to #4, this could be done via checked add or strict add when rust-lang/rust#118260 is resolved.