Extend #[must_use] to more types

[tmandry]

In particular, we should add #[must_use] to these "wrapper" types in the standard library:

• (request) Extend #[must_use] to Pin #67387 Pin
• Make Option<T> #[must_use] if T is #71368 Option

Both would be conditioned on whether T was #[must_use].

[Jasperav]

I support this! Furthermore, I like having must_have being the default. Explicitly opting out using the return value would IMO better, just like the way Swift is handling return values.

[tmandry]

This issue is only about adding #[must_use] to more "wrapper" types in the standard library and conditioning it on whether their type parameter is must_use. I'll update the description to make it more clear.