Rename filter.filter to filter.matches for clarity?

The "filter" method name is confusing because its not clear whether it means (is it that the filter filtered something out, or that we passed the criteria).

How about we rename [filter](https://github.com/pganalyze/pco_store/blob/8c7daac00b260b50a1e71e135ad7a49812be38af/src/lib.rs#L403) to "matches"? 