Added

- Log messages when a subscriber indicates that a span has closed, when the
  `log` feature flag is enabled (#180).

Changed

- `tracing-core` minimum dependency version to 0.1.2 (#174).

Fixed

- Fixed an issue where event macro invocations with a single field, using local
  variable shorthand, would recur infinitely (#166).
- Fixed uses of deprecated `tracing-core` APIs (#174).