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.

.github/workflows/fuzzing.yml

@@ -41,6 +41,8 @@ jobs:
     strategy:
       matrix:
         test-target:
+          - { name: fuzz_cmp, should_pass: true }
+          - { name: fuzz_cmp_args, should_pass: true }
           - { name: fuzz_ed, should_pass: true }
           - { name: fuzz_normal, should_pass: true }
           - { name: fuzz_patch, should_pass: true }