Hey, an idea... when dealing with ordered maps, it seems like it would be a common use case to be able to retrieve the index of an item by key, so that you can create subsets of the ordered map using slice() or similar.

Basically right now to slice an ordered map by key, I believe you have to iterate the key sequence first to figure out what index the key lives at, like so:

const index = orderedMap.keySeq().findIndex(k => k === key)

But this seems like it makes slice(), etc by key much slower. Although it's a tradeoff of manipulation time vs. write time.

Thoughts?