SizeHint tweaks

[cgzones]

The struct has only a size of 24 bytes and is trivially copy-able.

Permit SizeHint operations in const contexts and warn on unused
results from constructors or getters without side-effects.

[seanmonstar]

Thanks for this! I think making some of these functions const could be nice.

As for the #[must_use], these additions don't quite match up with normal Rust usage. You'll notice many things, even in libstd, return an inner value with no side-effects, and are not marked #[must_use]. It's usually only placed on specific types where forgetting to use the value is almost always wrong.

It looks like you might be interested in the allow-by-default unused-results lint, which you can enable for your projects.