Refactor the file layout

[JJ]

Right now CSV.pm (maybe it should be pm6 by now, or even rakumod) includes everything. And I mean everything. RangeSet, IO::String (?)

this should probably get a module all by itself

CVS::Row... Lots of things. It's not a big deal, but it might make stuff easier to document, handle and, well, refactor.

Also, lots of files in the root directory; they could probably be moved out of the way, to some "benchmarks" folder or some such. That would also help to check what we still need from them...

[Tux]

I like it in the same file, but this specific class could maybe do with its own file

[JJ]

IO::String is already out and refactored, file names are changed. I'll see if anything else can be done here and probably close.