commit.txt

This branch introduces: 

- A registry build atop of https://github.com/hawkw/sharded-slab. Layers
  are expected to consume this registry through the traits `SpanData`,
  `LookupSpan`, and `LookupMetadata`. Layer-specific data, such as
  formatted spans and events, are stored in the `Extensions` typemap. Data
  is writable via the `ExtensionsMut` view struct of the typemap. This
  enables layers to read and write data that they are specifically
  interested in.

- The `tracing_subscriber::fmt::Subscriber` has been re-implemented in
  terms of `tracing_subscriber::registry::Registry` and
  `tracing_subscriber::fmt::Layer`.

- The event/field formatters have been modified (in a non-backwards
  compatible way) to accept a `tracing_subscriber::fmt::FmtContext`. A
  similar structure existed in `tracing_subscriber::fmt::Subscriber`, but
  it was not publicly exposed.

Resolves #135 Resolves #157 Resolves #391

Signed-off-by: David Barsky <me@davidbarsky.com>
Coauthored-by: Eliza Weisman <eliza@buoyant.io>