library function to enumerate items by some filter

[ab9rf]

We have lots of places in dfhack where we get lists or counts of game items according to some filter. there are at least three different idioms being used for this, and several recently opened issues (#2418, #2417, #1473) have a commonality in that they involve using inadequately or improperly filtered item lists in some way.

This argues for a core library method, presuambly in Items, for doing item list filtering.