feat: add rebuild from existing function

[a10y]

It's useful to be able to rebuild a Compressor from an existing symbol table. For example, if you want to perform incremental compression, or compress a value to pushdown comparisons over the encoded data.

Going through CompressorBuilder, calling insert for each symbol and building may seem appropriate, but you're not guaranteed to get the same codes order afterward! This is because of the finalize step, many of the 2-byte codes will actually get reordered. This is an optimization we port from the C++ code, and allows us to skip a hashtable lookup during compression for many inputs.

To avoid the ambiguity, we provide a new Compressor::rebuild_from to rebuild from a slice of symbols and lens. This repopulates the codes_two_byte and hash table that are needed at compress time, and does so without the finalize step that occurs when going through the CompressorBuilder.

I've also gone and updated the fuzzer to check both a freshly built and a rebuilt compressor. It caught several bugs as I was going through cleaning up this PR 😅 I let the fuzzer run for 30 minutes on my laptop and it didn't find anything.

[codspeed-hq]

CodSpeed Performance Report

Merging #84 will not alter performance

_{Comparing aduffy/build-from-existing (187c4b5) with develop (44d9d78)}

Summary

✅ 16 untouched benchmarks

[a10y]

Just to tickle a fancy, I wanted to see if I could remove all of the unsafe without impacting performance, unfortunately it looks like, at least on microbenchmarks, removing unsafe has a substantial penalty.

Would be good future work though to see if this is possible.