Move responsibility to compute dots and by to method

[krlmlr]

Required for #6526.

Also fixes default argument of internal helper.

[DavisVaughan]

@krlmlr do you still need this one? Right now filter_rows() and slice_rows() are rather symmetrical, which I like, so I'd rather only do this if we have to.

We can definitely fix the bad default arg either way, of course

[krlmlr]

Good catch, slice_rows() has the same problem, fixed that 🤪

I've added comments to the code to explain the reasoning behind the separation.

[DavisVaughan]

@krlmlr would you please take a look at the CI?

[DavisVaughan]

Actually I see the problem, I'll take over and manage merging, thanks!

[krlmlr]

Thanks for taking over, I pushed too hastily.