This PR removes the public Deno.tracing.Span API.
We are not confident we can ship an API that is
better than the `@opentelemetry/api` API, because
V8 CPED does not support us using `using` to
manage span context. If this changes, we can
revisit this decision. For now, users wanting
custom spans can instrument their code using
the `@opentelemetry/api` API and `@deno/otel`.
This PR also speeds up the OTEL trace generation
by a 30% by using Uint8Array instead of
strings for the trace ID and span ID.
