Add the missing examples to the int trait

[mdinger]

std::num::Int is missing examples for these methods:
- [ ] Int::zero()
- [ ] Int::one()
- [ ] Int::min_value()
- [ ] Int::max_value()

• Int::saturating_add()
• Int::saturating_sub()

Examples for them should be added.

[steveklabnik]

Same as the other ticket: these are currently unstable.

[mdinger]

saturating_add() and saturating_sub() are stable. This should be open.