Add `cargo fuzz` target for `command` parser (and serializer).

fuzz/.gitignore

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+
+target
+corpus
+artifacts

fuzz/Cargo.toml

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+
+[package]
+name = "smtp-codec-fuzz"
+version = "0.0.0"
+authors = ["Automatically generated"]
+publish = false
+edition = "2018"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.4"
+
+[dependencies.smtp-codec]
+path = ".."
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[[bin]]
+name = "command"
+path = "fuzz_targets/command.rs"
+test = false
+doc = false

fuzz/fuzz_targets/command.rs

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+#![no_main]
+use libfuzzer_sys::fuzz_target;
+use smtp_codec::parse::command::command;
+
+fuzz_target!(|data: &[u8]| {
+    if let Ok((_, cmd)) = command(data) {
+        // Fuzzer created a valid SMTP command.
+        // dbg!(&cmd);
+
+        let cmd2 = {
+            // Let's serialize the command into bytes ...
+            let mut buf = Vec::new();
+            cmd.serialize(&mut buf).unwrap();
+
+            // ... parse it again ...
+            let (rem, cmd2) = command(&buf).unwrap();
+            assert!(rem.is_empty());
+
+            // dbg!(&cmd2);
+            cmd2
+        };
+
+        // ... and verify that we got the same results.
+        assert_eq!(cmd, cmd2);
+    }
+});