v0.4.0

Added

• Feature tokio (off by default) adds serde_markdown::tokio, the async
  twin of every function under the same name: to_string,
  to_string_with_format, to_string_with, to_vec, to_writer (tokio
  AsyncWrite), from_str, from_slice, from_reader (tokio
  AsyncRead). It writes and reads the same documents as the sync form. It
  is native: the same steps, with a cooperative yield between them when the
  task's budget is spent; no thread and no spawn_blocking, so an async
  caller awaits it as it is.
• The runnable example example-tokio (rust/examples/tokio): the async
  form on a tokio runtime, with a file round-trip over tokio::fs and
  pages rendered in parallel tasks.

Changed

• The README is rewritten for users of the crate: how to depend on it, the
  document shape, body fields, language and layout, errors, the async API
  (with a file round-trip over tokio::fs), and protobuf messages; the
  development notes moved to the end. Its examples are checked against the
  crate.
• When a required body section and front-matter keys are both missing,
  the error is the first missing field in declaration order:
  ErrorKind::Type for a front-matter key. Before, the missing body
  section (ErrorKind::Body) was reported first.
• markdown/options.proto, the (markdown.body) option, moved into the crate:
  rust/markdown/proto/markdown/options.proto. The import path is the same,
  markdown/options.proto. The Cargo dependency now brings the option too, so
  a consumer needs no second channel for it.
• PROTO_INCLUDE points into the crate (<crate>/proto), so it works for a
  crate fetched alone, not only from a checkout of this repository.

Removed

• The Bazel label @serde_markdown//proto/markdown:options.proto. A Bazel
  consumer drops its bazel_dep and git_override of serde_markdown and
  reaches the file through its crate_universe hub with an annotation
  (additive_build_file_content plus extra_aliased_targets); the README
  shows it. With one dependency the proto and the crate can no longer drift
  to different versions.
• The fixtures crate serde_markdown-proto (not published) no longer carries
  the generated markdown.body extension constant.

Fixed

• Enum fields read back. A field of an enum type (Serde's default,
  externally tagged form) failed to read with ErrorKind::Type, in the
  fields block and in a body section. Unit variants and variants with
  content now round-trip in YAML, JSON, and TOML; a unit variant in a body
  section is its raw name.
• #[derive(Markdown)] follows the struct's #[serde(rename_all = "...")]
  (and rename_all(serialize = ..., deserialize = ...)) and drops the r#
  of a raw identifier. Before, a body field of such a struct was written
  into the fields block, with no error. A body field whose serialize and
  deserialize names differ is now a compile error.
• A Vec<u8> body whose text reads as a YAML or JSON list ([1, 2],
  - a) reads back as its bytes; before, it read as the list's numbers or
  failed. A Vec<u8> whose text reads as an empty list ([]) is
  ErrorKind::Body on write, since it cannot be told from an empty list.
• A body field with #[serde(default)] may be missing from the document:
  a field without a section is no longer handed to the value, so Serde
  applies its default (or None). A required one is still
  ErrorKind::Body.
• A TOML date-time in the fields block (when = 1979-05-27T07:32:00Z)
  reads into a string or Timestamp field; before, ErrorKind::Type.
• Built without the yaml feature, a document whose first slice is prose
  reads as body, and a body-only document is written with no empty fields
  block. Before, reading failed with ErrorKind::FormatDisabled and writing
  added an empty JSON or TOML block.
• Leading --- lines are skipped in linear time. A document of many of
  them took quadratic time (40 000 lines: 1.4 s).
• Every feature set builds and tests, none at all included.