Switch to a custom Myers-based LCS engine

BENCHMARKING.md

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+# Benchmarking diff
+
+The engine used by our diff tool tries to balance execution time with patch
+quality. It implements the Myers algorithm with a few heuristics which are also
+used by GNU diff to avoid pathological cases.
+
+The original paper can be found here:
+- https://link.springer.com/article/10.1007/BF01840446
+
+Currently, not all tricks used by GNU diff are adopted by our implementation.
+For instance, GNU diff will isolate lines that only exist in each of the files
+and not include them on the diffing process. It also does post-processing of the
+edits to produce more cohesive hunks. Both of these combinar should make it
+produce better patches for large files which are very different.
+
+Run `cargo build --release` before benchmarking after you make a change!
+
+## How to benchmark
+
+It is recommended that you use the 'hyperfine' tool to run your benchmarks. This
+is an example of how to run a comparison with GNU diff:
+
+```
+> hyperfine -N -i --warmup 2 --output=pipe 'diff t/huge t/huge.3'
+'./target/release/diffutils diff t/huge t/huge.3'
+Benchmark 1: diff t/huge t/huge.3
+  Time (mean ± σ):     136.3 ms ±   3.0 ms    [User: 88.5 ms, System: 17.9 ms]
+  Range (min … max):   131.8 ms … 144.4 ms    21 runs
+
+  Warning: Ignoring non-zero exit code.
+
+Benchmark 2: ./target/release/diffutils diff t/huge t/huge.3
+  Time (mean ± σ):      74.4 ms ±   1.0 ms    [User: 47.6 ms, System: 24.9 ms]
+  Range (min … max):    72.9 ms …  77.1 ms    41 runs
+
+  Warning: Ignoring non-zero exit code.
+
+Summary
+  ./target/release/diffutils diff t/huge t/huge.3 ran
+    1.83 ± 0.05 times faster than diff t/huge t/huge.3
+>
+```
+
+As you can see, you should provide both commands you want to compare on a single
+invocation of 'hyperfine'. Each as a single argument, so use quotes. These are
+the relevant parameters:
+
+- -N: avoids using a shell as intermediary to run the command
+- -i: ignores non-zero exit code, which diff uses to mean files differ
+- --warmup 2: 2 runs before measuring, warms up I/O cache for large files
+- --output=pipe: disable any potential optimizations based on output destination
+
+## Inputs
+
+Performance will vary based on several factors, the main ones being:
+
+- how large the files being compared are
+- how different the files being compared are
+- how large and far between sequences of equal lines are
+
+When looking at performance improvements, testing small and large (tens of MBs)
+which have few differences, many differences, completely different is important
+to cover all of the potential pathological cases.