Add benchmarking with CI job - #9

Hey @vvish thanks for the PR. I'm also looking into adding benchmarks in #8 where I also refactor to share some factories between test and benchmarking.

There I have added a map_unary (yet do do binary / n-ary) that significantly overlap with transform.
The idea is for it to be more generic and free from the STL apis constraints (leverage alignment knowledge, purity, also no macros), handle more complex functions (e.g. xsimd::widen) and different type widths.
It seems to perform a bit better on small and medium runs. Ideally transform would become a special case of it.
Curious to know what you think.

As for benchmarking in CI, the setup suggested here will be extremely noisy. We plan to add the same approach as xtensor with namespace+codspeed.