core: add Subscriber::try_close

[hawkw]

Motivation

As discussed in #136, a proposed Layer trait for composing
subscribers required should probably not be responsible for
managing ref-counts and determining span closures. Instead, the root
subscriber should be responsible for this, and Layers should be able
to opt into a callback that's notified when a span is closed.

Adding a callback that's called on closure to Layer requires modifying
the Subscriber trait to allow indicating when a span closes.

Solution

This branch attempts to do this without a breaking change, by adding a new
try_close trait method to Subscriber. try_close is identical to
drop_span except that it returns a boolean value, set to true if the
span has closed.

The try_close default implementation simply returns false, so that
if subscribers don't care about tracking ref counts, close notifications
will never be triggered. The default implementation of drop_span is
changed to call self.try_close(...) and ignore the return value.

The drop_span documentation now indicates that it is "soft deprecated"
similarly to std::error::Error's description method. This encourages
subscribers to implement the new API, but isn't a breaking change.
Existing callers of drop_span are still able to call it without
causing issues, as the Layered::drop_span will always call try_close
on the inner subscriber, in order to forward the closure notification to
the layer if the span closes.

[hawkw]

@jonhoo I've changed try_close to call drop_span, as you suggested in #153 (comment), and updated the documentation a bit. It would be great to get your approval on this branch, as I'd like to move forward with this.

[jonhoo]

@hawkw happy to approve subject to the one comment above.