[no_std] compatibility for standalone crates

[jschneider-bensch]

E.g. for libcrux-kem, or in general where possible.

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[franziskuskiefer]

@jschneider-bensch let's list the crates and what needs to get done.

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This issue has been marked as stale due to a lack of activity for 60 days. If you believe this issue is still relevant, please provide an update or comment to keep it open. Otherwise, it will be closed in 7 days.

[franziskuskiefer]

Assigning to @keks as part of the first iteration of a pure Rust libcrux #648.