Create utils to transform between selection range and query

[severo]

propose utility functions to transform (in both directions) between a sorting aware selection (rows 0 -> N, sorted by score) and a filter (score < 10). There is an ambiguity that must be removed (through UI? through some rules?): if we select the first N rows, are we applying WHERE score < 10 or ORDER BY score LIMIT {N}?

from #103 (comment)