message: Add favicon to essential files
  This can be tested with `cargo run build add-essential-files && cargo
  build chrono 0.4.18`.
diff: @@ -26,6 +26,7 @@ use std::sync::Arc;
  const USER_AGENT: &str = "docs.rs builder (https://github.com/rust-lang/docs.rs)";
  const ESSENTIAL_FILES_VERSIONED: &[&str] = &[
  "brush.svg",
  + "favicon.svg",
  "wheel.svg",
  "down-arrow.svg",
  "dark.css",
  