use zlib-rs for gzip compression in rust code

Various C dependencies (curl, git) still rely on `libz-sys`, so for the time being a system libc is still required. But, using zlib-rs via flate2 is straightforward, and gives good speedup for `cargo package`. It is also extremely portable, because it's just rust code.

Author: folkertdev

Cargo.lock

@@ -45,7 +45,7 @@ criterion = { version = "0.5.1", features = ["html_reports"] }
 curl = "0.4.47"
 curl-sys = "0.4.79"
 filetime = "0.2.25"
-flate2 = { version = "1.0.35", default-features = false, features = ["zlib"] }
+flate2 = { version = "1.1.1", default-features = false, features = ["zlib-rs"] }
 git2 = "0.20.0"
 git2-curl = "0.21.0"
 gix = { version = "0.70.0", default-features = false, features = ["blocking-http-transport-curl", "progress-tree", "parallel", "dirwalk"] }

Cargo.toml

@@ -57,7 +57,7 @@ The build will automatically use vendored versions of the following libraries. H
 * [`libcurl`](https://curl.se/libcurl/) — Used for network transfers.
 * [`libgit2`](https://libgit2.org/) — Used for fetching git dependencies.
 * [`libssh2`](https://www.libssh2.org/) — Used for SSH access to git repositories.
-* [`libz`](https://zlib.net/) (aka zlib) — Used for data compression.
+* [`libz`](https://zlib.net/) (AKA zlib) — Used by the above C libraries for data compression. (Rust code uses [`zlib-rs`](https://github.com/trifectatechfoundation/zlib-rs) instead.)
 
 It is recommended to use the vendored versions as they are the versions that are tested to work with Cargo.
 

README.md

@@ -187,7 +187,7 @@ The [`default` features](#the-default-feature) can be disabled using
 
 ```toml
 [dependencies]
-flate2 = { version = "1.0.3", default-features = false, features = ["zlib"] }
+flate2 = { version = "1.0.3", default-features = false, features = ["zlib-rs"] }
 ```
 
 > **Note**: This may not ensure the default features are disabled. If another