Move GEMM benchmark to nanobench

[Mousius]

This is a proof of concept, for moving the benchmark suite to use nanobench, which is a single header file but has advantages over the existing benchmark suite:

• Robustness - nanobench will explicitly tell you when benchmarks are unstable, and offer some guidance to help stabilise them.
• JSON Output - this should allow us to store the results in a more machine friendly format, in future we can integrate this into CI to do checks on PRs

I also looked at Google Benchmark and Catch2, but nanobench was more appealing due to minimal integration requirements with the existing hybrid build architecture.