Fix span linking

[ThomWright]

This took me quite a number of hours with the debugger to figure out.

The key parts are:

• Spans can be cloned. Much like Arc, they have a reference count.
• When a span gets dropped, an attempt will be made to close it. It will only close it if the ref count is zero.
• Closing a span removes it from the Registry.
• If you try to call span_a.follows_from(span_b_id) after span B has been closed, it will panic. Therefore, a Span with that ID must still be in scope.
• If you call span_a.follows_from(span_b) then span B will be consumed (and hence dropped) and converted into an ID before being passed to the function. This ID will no longer refer to a valid span in the Registry.

The following both work:

• Passing by reference: span_a.follows_from(&span_b)
• Passing just the ID, keeping the span alive: span_a.follows_from(span_b.id())