Add must_use function attributes

[cgzones]

Annotate several public funtions that either don't modify their original value or only compute a value, except for ones returning Result.

Avoid no-op code such as:

let odt = OffsetDateTime::now_utc();
odt.saturating_add(Duration::SECOND);

[jhpratt]

If there is a specific method you think could be reasonably interpreted as mutating the input that does not already have a #[must_use] attribute, please open an issue to discuss it. Adding it to everything is functionally pointless.

[cgzones]

Is it though?
For example std::time::Duration uses the annotation on almost every interface (except for ones returning Result, since Result already comes with an annotation).

[jhpratt]

And I disagree with using it that much. As I said, a method needs to be able to be reasonably interpreted as mutating the input. That is the criteria that I am using. Anything else is a non-starter.