Allow `ns` to have run-time defined namespaces

[ValarDragon]

Summary

Namespace names at the moment must be &'static str. There are use cases in which you would want runtime defined namespaces, e.g. for generating many circuits of different sizes, or prefixing what step of a computation you are in.

I think we should try to generalize this to &str. I'm not really clear why its limited to &'static str at the moment, is this imposed by tokio-rs/tracing's info_span! macro?

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[Pratyush]

Yes, the limitation is due to tracing. There might be a way to work around it though