Add a feature enabling native zlib for cache entries.

This is just one of a few things to try related to #552. As a first step it
would be interesting to see what kind of impact this has on Firefox automation
builds. Some initial measurements are promising for builds with lots of cache
misses, but this sort of thing is cumbersome to measure, so I'd like to try
enabling this for the sccache client built in Mozilla automation to see the
impact it has there.

Author: chmanchester

Cargo.lock

@@ -132,6 +132,7 @@ s3 = ["chrono", "hyper", "hyperx", "reqwest", "simple-s3", "hmac", "sha-1"]
 simple-s3 = []
 gcs = ["chrono", "hyper", "hyperx", "reqwest", "untrusted", "url"]
 memcached = ["memcached-rs"]
+native-zlib = ["zip/deflate-zlib"]
 # Enable features that require unstable features of Nightly Rust.
 unstable = []
 # Enables distributed support in the sccache client