Add tracing integration

[JoeyBF]

This PR adds optional support for the tracing crate, as mentioned in #915 (especially #915 (comment)).

Spans (INFO level):

• rayon::worker_thread -- Wraps each worker thread's lifetime. Fields: worker, pool_id
• rayon::job_execute -- Wraps each job execution. Fields: job_id, worker

Events (DEBUG level):

• rayon::thread_idle -- Worker going idle
• rayon::thread_active -- Worker waking up
• rayon::job_injected -- Job injected into global queue. Fields: job_id, pool_id
• rayon::job_stolen -- Job stolen from another thread. Fields: job_id, victim

Jobs capture the current span context when created. When executed (potentially on a different thread), they restore this context so job_execute spans appear as children of the span that spawned the work. This enables proper distributed tracing across thread boundaries.

Furthermore, all instrumentation compiles to no-ops when the feature is disabled, JobContext is zero-sized when tracing is off, and field values are computed inside macros to avoid overhead when filtered out.