After long discussions with @ELLIOTTCABLE, the collector should only
provide a `with_`-like function for regular spans. This allows
each concurrency library to use its native mechanism for local storage,
which is almost always based on such a scope function.
These spans require the user to pass the surrounding span, if any, when
entering a new span. They use the information inside (which is collector
specific) to track what asynchronous task is currently being executed.
Wrappers around `trace`, specific to an async library (e.g. Eio, lwt,
Async, etc.) can then smooth this over by providing a `with_async_span`
construct that uses some implicit contextual storage to carry the
`surrounding` scope around.
this way, we can actually use only `trace.core` when we also expect to
use compiler-libs.toplevel (which contains a naked `Trace` module, which
would cause .cmi collisions). The library `trace` still exists, and just
forwards to `trace.core`.
