Add benchmarks

It'd be great to have benchmarks on lens-csv; bonus points if they compare performance against cassava directly 😄

If it's possible to profile memory usage to ensure that streaming from disk is working, that'd be a bonus

The issue names no files, tests, or benchmark entry points. Start by inspecting the repository structure and build configuration for existing test or benchmarking setup, then establish measurements for lens-csv and a comparison with cassava. Done means reproducible benchmark results; memory profiling is an optional bonus mentioned by the issue.

Written by the indexing model from the issue text.