o Major features (Rust experimentation):
    - Tor now ships with an optional implementation of one of its smaller
      modules (protover.c) in the Rust programming language.  To try it
      out, install a Rust build environment, and configure Tor with
      "--enable-rust --enable-cargo-online-mode".  This should not
      cause any user-visible changes, but should help us gain more experience
      with Rust, and plan future Rust integration work.
      Implementation by Chelsea Komlo. Closes ticket 22840.