Switch flate2 to use the zlib-rs backend

This backend is written in Rust, but has substantially higher
performance. And, as of the latest version, zlib-rs does not
export any C zlib symbols, so it won't conflict with other
implementations of zlib in the same process; thus, it doesn't conflict
with linking to C libraries that use zlib.

Author: joshtriplett

Cargo.lock

@@ -50,7 +50,7 @@ curl = { version = "0.4.44", optional = true }
 effective-limits = "0.5.5"
 enum-map = "2.5.0"
 env_proxy = { version = "0.4.1", optional = true }
-flate2 = "1"
+flate2 = { version = "1.1.1", default-features = false, features = ["zlib-rs"] }
 fs_at = "0.2.1"
 git-testament = "0.2"
 home = "0.5.4"