`[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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[franziskuskiefer]

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

[keks]

I made most crates no_std in #697. Some didn't work, though:

• sys/hacl: depends on std::os
• psq: uses std::time::SystemTime
• macros: is proc-macro, so shouldn't need to

The PSQ one might be solvable, but I think our chances for sys/hacl are not very good. But since we are already in the way to migrating to hacl-rs, that is temporary.

@franziskuskiefer if you don't have objections, I would close this as resolved.

[franziskuskiefer]

• sys/hacl: depends on std::os

As you say, since this is for wrapping C code we don't care for the pure Rust version.

• psq: uses std::time::SystemTime

Let's not put this in scope for the first pure Rust release and file a separate issue for time not being in core.

• macros: is proc-macro, so shouldn't need to

Right, this is fine.

Let's file that psq issue and close this.