Add withSpan overloads for automatic context propagation

[slashmo]

We should add withSpan overloads which take no Baggage argument, and instead use the task local Baggage.current. We'll need one synchronous and one async version (at least until reasync was added to the language). These overloads should execute the given operation within a child task where Baggage.$current is bound to the newly created span's Baggage. That way spans started within that operation will automatically become children of the outer span (depending on the tracer implementation).