Certain headers require `optional<T&>`, so it may be beneficial to split
out `optional<T>` and `optional<T&>`. This can allow consumers to only
bring in the `optional` flavour it needs..
- Parcel out the respective pieces into their own header.
- Certain sources rely on transitive includes brought in by
`<optional>`, so they're kept there for now, and only tests have been
fixed.
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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
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