* tracing: update core version

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

* tracing: prepare to release 0.1.12

Added

- `Span::with_subscriber` method to access the subscriber that tracks a
  `Span` (#503)
- API documentation now shows which features are required by
  feature-flagged items (#523)
- Improved README examples (#496)
- Documentation links to related crates (#507)

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

* Update CHANGELOG.md