Remove rental crate and implement the required functionality

[mattrutherford]

Closes #6312

Introduces new struct ProxiedSpan that replaces SpanAndGuard that was used in conjunction with rental.

This allows us to remove the now unmaintained rental crate.

[mattrutherford]

This new code essentially replicates what the rental crate was doing, just with a strictly limited interface. I think it should soon be possible to replace the need for this altogether, so perhaps it's worth leaving things as is with the rental crate for now. However I don't think there would be any downside to merging this, if deemed desirable.

[bkchr]

@mattrutherford Do we still need this?

[mattrutherford]

It's uncertain, but possible that we still need to do this for WASM, because as of right now, experiments to access the spans directly via a new interface breaks the relationship between spans from wasm and corresponding native tracing spans/events. It's possible to avoid this using our custom subscriber if we switch to use 2 Events (enter/exit) instead of a single Span and patch them to our span representation in the subscriber, but that wouldn't preserve the parent_id relationships if we adopt the tracing_subscriber::FmtSubscriber in future.

However maybe OK to close this PR for now seeing as there's no WASM tracing implementation currently using it yet - we can bring these changes into a future PR that introduces it if still needed by then.