o Major features (directory system):
    - Tor's compression module now includes support for the zstd and lzma2
      compression algorithms, if the libzstd and liblzma libraries are
      available when Tor is compiled.  Once these features are exposed in the
      directory module, they will enable Tor to provide better compression
      ratios on directory documents. Part of an implementation for proposal
      278; closes ticket 21662.

  o Code simplification and refactoring:
    - Tor's compression module now supports multiple backends. Part of
      an implementation of proposal 278; closes ticket 21663.