Simpler API for table outputs

[Julow]

I wanted to save the benchmark results into a CSV and found that it was quite difficult to deal with the current API.

The code in the examples folder uses the merge function, which has a confusing signature. On top of that, it looses the list of test names, which makes creating a CSV output harder and error prone.

I struggled quite a bit to get a correct CSV output, so I figured that it would be worthwhile to support this usecase in the library.

Maybe related to https://github.com/mirage/bechamel/issues/53