one can now give tasks an optional string "name". If `Trace`
is present (installed) and enabled, this results in
a span around the task's execution. This also plays ok
with `await` and other effect-based primitives.
futures, chans, etc. should use the `Runner.t` abstraction
and not depend on the exact pool implementation to run tasks.
For now `Pool.create` is the only implementation of a runner, but now
it's possible to implement alternatives.
