Support use_tomap everywhere

[DifferentialOrange]

crud.pairs has use_tomap option allowing to return tuples as key-value maps instead of arrays, since there is no way to do unflatten_rows while calling pairs.

But this feature seems useful by itself. @WeCodingNow had reported that he uses crud.pairs():totable() instead of crud.select since it is impossible to write an oneliner that returns tuples as key-value maps with crud.select. There doesn't seem to be any harm in implementing use_tomap, so let's support it.