Add safe Rust Zstd decoding option

Cargo.toml

@@ -25,6 +25,7 @@ fax34 = { package = "fax", version = "0.2.6", optional = true }
 flate2 = { version = "1.0.20", optional = true }
 weezl = { version = "0.1.10", optional = true }
 zstd = { version = "0.13", optional = true }
+ruzstd = { version = "0.8.2", optional = true }
 zune-jpeg = { version = "0.5.1", optional = true }
 image-webp = {version = "0.2.4", optional = true }
 
@@ -45,6 +46,9 @@ jpeg = ["dep:zune-jpeg"]
 lzw = ["dep:weezl"]
 webp = ["dep:image-webp"]
 zstd = ["dep:zstd"]
+# Fully safe Rust but much slower Zstd decoding.
+# When both zstd and safe-rust-zstd are enabled, zstd takes precedence.
+safe-rust-zstd = ["dep:ruzstd"]
 
 [[bench]]
 name = "lzw"