Add clone_span and drop_span hooks to Subscriber

[hawkw]

Closes #80

This branch adds new clone_span and drop_span methods to the Subscriber
trait, which are called when a span handle is cloned or dropped, respectively.
This allows the subscriber to track the number of span handles with a given ID,
so that a pointer (or an index into an arena) may be used as span IDs, and the
subscriber may deallocate the storage for a span when that span no longer
exists.

Both of these methods have default implementations which do nothing, so that
subscriber implementations which don't need to track this information don't need
to implement them. However, they may be overridden by subscribers which do care
about span reference counts. This is similar to the clone_id and drop_id methods of
futures::executor::Notify.

This branch also allows the test support mock subscriber to expect calls to
these methods, and adds new unit tests for them.

Signed-off-by: Eliza Weisman eliza@buoyant.io