fix(zstd): fastest level now does compression

Author: robjtede

src/lib.rs

@@ -254,6 +254,12 @@ impl Level {
     #[cfg(feature = "zstd")]
     fn into_zstd(self) -> i32 {
         let (fastest, best) = libzstd::compression_level_range().into_inner();
+
+        // NOTE: zstd's "fastest" level is -131072 which can create outputs larger than inputs.
+        // This library chooses a "fastest" level which has a more-or-less equivalent compression
+        // ratio to gzip's fastest mode. We still allow precise levels to go negative.
+        // See discussion in https://github.com/Nullus157/async-compression/issues/352
+
         match self {
             Self::Fastest => fastest,
             Self::Best => best,