fix Rust through CMake

[lemire]

This is an alternative way to benchmark against Rust. Except for the fact that Rust should be available on the machine (automatically so if you use the suggested docker container), it works automagically.

I also fixed the nlohmann_json benchmark for twitter since it did not appear to be doing much. I think I have something fairer now.

I am not 100% sure that the Rust benchmark is equivalent though I tried my best. There is a bunch of indices_start, indices_end everywhere, but they do not appear in the JSON that we are parsing. I did not want to remove them because I just don't understand what they are, but they are subject to removal. It would be nicer if the task was well defined and equivalent throughout, for all functions.

This PR is an alternative to #41

Relates to #40

[FranciscoThiesen]

Thanks for this. I'll work to make sure that rust benchmark is indeed equivalent to the C++ ones.