Add tests for write performance

[pjfsilva]

I do understand the goal of this project and the tests being done, but I think a nice addition (that makes sense in this context) would be to have a new benchmark for write performance on libraries that support it. Or maybe that's a different project for anyone to take :)

[MarkPflug]

If you're interested in write performance, I have some benchmarks that measure that for a handful of libraries, which you can find here: https://github.com/markpflug/benchmarks