Add #[must_use] attribute to HashMap and HashSet constructors

[xtqqczze]

• new_in
• with_capacity_and_hasher
• with_capacity_and_hasher_in
• with_hasher
• with_hasher_in

See also: #89692

[rustbot]

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[joboet]

r? libs-api
but this makes sense to me.

[xtqqczze]

@joboet perhaps if the return type name for these functions was specified as Self this would be helpful for review?

[joboet]

No, on the contrary; having this PR only contain the #[must_use] one-line additions is rather helpful.