Add cmp utility

The utility should support all the arguments supported by GNU cmp and
perform slightly better.

On a "bad" scenario, ~36M files which are completely different, our
version runs in ~72% of the time of the original on my M1 Max:

 > hyperfine --warmup 1 -i --output=pipe \
     'cmp -l huge huge.3'
 Benchmark 1: cmp -l huge huge.3
   Time (mean ± σ):      3.237 s ±  0.014 s    [User: 2.891 s, System: 0.341 s]
   Range (min … max):    3.221 s …  3.271 s    10 runs

   Warning: Ignoring non-zero exit code.

 > hyperfine --warmup 1 -i --output=pipe \
     '../target/release/diffutils cmp -l huge huge.3'
 Benchmark 1: ../target/release/diffutils cmp -l huge huge.3
   Time (mean ± σ):      2.392 s ±  0.009 s    [User: 1.978 s, System: 0.406 s]
   Range (min … max):    2.378 s …  2.406 s    10 runs

   Warning: Ignoring non-zero exit code.

Our cmp runs in ~116% of the time when comparing libxul.so to the
chromium-browser binary with -l and -b. In a best case scenario of
comparing 2 files which are the same except for the last byte, our
tool is slightly faster.

fuzz/Cargo.toml

@@ -16,6 +16,13 @@ diffutils = { path = "../" }
 [workspace]
 members = ["."]
 
+[[bin]]
+name = "fuzz_cmp"
+path = "fuzz_targets/fuzz_cmp.rs"
+test = false
+doc = false
+
+
 [[bin]]
 name = "fuzz_patch"
 path = "fuzz_targets/fuzz_patch.rs"

fuzz/fuzz_targets/fuzz_cmp.rs

@@ -0,0 +1,51 @@
+#![no_main]
+#[macro_use]
+extern crate libfuzzer_sys;
+use diffutilslib::cmp::{self, Cmp};
+
+use std::ffi::OsString;
+use std::fs::File;
+use std::io::Write;
+
+fn os(s: &str) -> OsString {
+    OsString::from(s)
+}
+
+fuzz_target!(|x: (Vec<u8>, Vec<u8>)| {
+    let args = vec!["cmp", "-l", "-b", "target/fuzz.cmp.a", "target/fuzz.cmp.b"]
+        .into_iter()
+        .map(|s| os(s))
+        .peekable();
+
+    let (from, to) = x;
+
+    File::create("target/fuzz.cmp.a")
+        .unwrap()
+        .write_all(&from)
+        .unwrap();
+
+    File::create("target/fuzz.cmp.b")
+        .unwrap()
+        .write_all(&to)
+        .unwrap();
+
+    let params =
+        cmp::parse_params(args).unwrap_or_else(|e| panic!("Failed to parse params: {}", e));
+    let ret = cmp::cmp(&params);
+    if from == to && !matches!(ret, Ok(Cmp::Equal)) {
+        panic!(
+            "target/fuzz.cmp.a and target/fuzz.cmp.b are equal, but cmp returned {:?}.",
+            ret
+        );
+    } else if from != to && !matches!(ret, Ok(Cmp::Different)) {
+        panic!(
+            "target/fuzz.cmp.a and target/fuzz.cmp.b are different, but cmp returned {:?}.",
+            ret
+        );
+    } else if ret.is_err() {
+        panic!(
+            "target/fuzz.cmp.a and target/fuzz.cmp.b caused cmp to error ({:?}).",
+            ret
+        );
+    }
+});