message: Don't expose a markdown library
  Summary: Starlark shouldn't be providing a library of markdown functions in its public API. Hide all apart from the two that are useful, and move those two so they don't appear in a markdown module.
diff: // TODO(nga): document it
  #![allow(missing_docs)]
  -pub mod markdown;
  +mod markdown;
  use std::collections::HashMap;
  use allocative::Allocative;
  use gazebo::prelude::*;
  use itertools::Itertools;
  +pub use markdown::AsMarkdown;
  +pub use markdown::MarkdownFlavor;
  use once_cell::sync::Lazy;
  use regex::Regex;
  use regex::RegexBuilder;
  