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

See https://bugs.torproject.org/21663

See https://bugs.torproject.org/21663

This patch changes the way `tor_compress_new()`,
`tor_compress_process()`, and `tor_compress_free()` handles different
compression methods. This should give us compiler warnings in case an
additional compression method is added, but the developer forgets to add
handlers in the three aforementioned functions.

See https://bugs.torproject.org/21663