Since we have a streaming API for each compression backend, we don't
need a non-streaming API for each: we can build a common
non-streaming API at the front-end.
This patch adds support for checking if a given `compress_method_t` is
supported by the currently running Tor instance using
`tor_compress_supports_method()`.
See: https://bugs.torproject.org/21662
This patch refactors the `torgzip` module to allow us to extend a common
compression API to support multiple compression backends.
Additionally we move the gzip/zlib code into its own module under the
name `compress_zlib`.
See https://bugs.torproject.org/21664