Add examples for Checked[Add|Sub|Mul|Div]

[nham]

No description provided.

[alexcrichton]

In this example and the examples below I think you should be able to leave off the suffix on literals except for the receiver of the checked_foo method. In other words, when unwrapping I don't think you'll need the suffix and when passing arguments I don't think you'll need the suffix.

[lilyball]

The equality assert should be simplified to an assert_eq (which also lets us drop the variable), as in

assert_eq!(5u16.checked_add(&65530), Some(65535));

[nham]

Good call. I'm not sure why I added those.

[lilyball]

Aesthetically, I'd prefer it if the is_none() asserts were also converted to assert_eq!() (because it would line up nicely with the previous assert_eq!(), but otherwise, LGTM.

[nham]

Done

[lilyball]

LGTM. r=me if you squash the commits.

[brson]

Thanks for the docs!