Add #where support for MemoryScope.

An `ArgumentError` (to me) signals misuse of a public API. Here there is no such *misuse* we raise in case a specific feature was requested that does not exist yet.

As a compromise I think we could subclass and raise `ArgumentError` into `FinderMethodNotImplementedError`. Best of two worlds IMO.

I as a naive user of `MemoryScope#where` want to differentiate between errors that are from my improper usage of AR or the limits of `MemoryScope`.