tracing-subscriber v0.3.0

This is a breaking release of tracing-subscriber. The primary breaking
change in this release is the removal of the dependency on the chrono
crate, due to RUSTSEC-2020-0159. To replace chrono, support is
added for formatting timestamps using the time crate instead.

In addition, this release includes a number of other breaking API
changes, such as adding (limited) support for #![no_std] targets,
removing previously deprecated APIs, and more.

Breaking Changes

• Removed APIs deprecated in the v0.2.x release series.
• Renamed Layer::new_span to Layer::on_new_span (#1674)
• Removed Layer impl for Arc<L: Layer<S>> and Arc<dyn Layer<S> + ...>
  (#1649)
• Replaced the chrono crate with the time crate for timestamp
  formatting, to resolve RUSTSEC-2020-0159 (#1646)
• Removed json and env-filter from default features. They must now
  be enabled explicitly (#1647). This means that RUST_LOG-based filters will not
  work unless the env-filter feature is enabled.
• Changed FormatEvent::format_event and FormatFields::format_fields
  trait methods to take a Writer type, rather than a &mut dyn fmt::Write trait object (#1661)
• Changed the signature of the MakeWriter trait by adding a lifetime
  parameter (#781)

Changed

• layer: Renamed Layer::new_span to Layer::on_new_span (#1674)
• fmt: Changed FormatEvent::format_event and
  FormatFields::format_fields trait methods to take a Writer type,
  rather than a &mut dyn fmt::Write trait object (#1661)
• json, env-filter: json and env-filter feature flags are no
  longer enabled by default (#1647)

Removed

• Removed deprecated CurrentSpan type (#1320)

• registry: Removed deprecated SpanRef::parents iterator, replaced
  by SpanRef::scope in [#1431] ([#1648)])

• layer: Removed deprecated Context::scope iterator, replaced by
  Context::span_scope and Context::event_scope in #1431 and
  #1434 (#1648)

• layer: Removed Layer impl for Arc<L: Layer<S>> and Arc<dyn Layer<S> + ...>. These interfere with per-layer filtering. (#1649)

• fmt: Removed deprecated LayerBuilder type (#1673)

• fmt: Removed fmt::Layer::on_event (renamed to
  fmt::Layer::fmt_event) (#1673)

• fmt, chrono: Removed the chrono feature flag and APIs for
  using the chrono crate for timestamp formatting (#1646)

  Added

• fmt, time: LocalTime and UtcTime types for formatting
  timestamps using the time crate (#1646)

• fmt: Added a lifetime parameter to the MakeWriter trait,
  allowing it to return a borrowed writer. This enables implementations
  of MakeWriter for types such as Mutex<T: io::Write> and
  std::fs::File. (#781)

• env-filter: Documentation improvements (#1637)

• Support for some APIs on #![no_std] targets, by disabling the std
  feature flag (#1660)

Thanks to @Folyd and @nmathewson for contributing to this release!