Benchmark for Yojson

These benchmarks require Core_bench which is not a dependency of Yojson, because it is not part of the regular installation/testing flow. This is solely meant for developers that are worried about performance changes in Yojson.

This is also why it is deemed appropriate to depend on a benchmarking library with this many dependencies: we rather spend time writing benchmarks than to build a benchmarking library if a perfectly good one already exists in OCaml.