Indexing unit benchmarks

[ManyTheFish]

Summary

Add a unit benchmark for each extractor in the indexer, the goal is:

• during the development phase, seeing fastly and efficiently the impact of local optimization
• during the performance-regression investigation phase, helping to find local regressions

Checklist

• extractors
• prefix db

[irevoire]

I started looking into this issue and it’s not as easy as I thought;

• criterion does not support it
• bencher does not support it
• microbench do not use the #[bench] attribute and thus will run everytime we run our tests
• It’s still nightly in the stdlib and we can’t do a #[cfg(nightly)] but it may be possible to make something ok-ish with select-rustc

[irevoire]

I implemented a first version of a unit benchmark using the nightly stdlib solution since it’s the only one that allows unit benchmarks.
You can look at this commit; meilisearch/milli@10b42cc
Basically, we need to import select-rustc; once this is done, we can #[cfg(feature = "nightly")], and we need to do this for everything related to the benchmarks.
To simplify the code a little bit, I created a module in the test module where I threw my benchmarks.