Actually use cache for newly-constructed VirtualArrays

[nsmith-]

Also require cache to conform to MutableMapping interface, i.e. KeyError
is the only exception that indicates trouble.

[jpivarski]

I had a question about one line, but this looks right. Give me a checkmark or thumbs up or something and I'll merge it.

[nsmith-]

My one other complaint is there's a lot of key in cache checks that may be optional if we trust the virtual array to have correct type before materialization. This isn't a huge deal, but there may be unnecessary overhead in the cases where the cache is backed by something slow.

[jpivarski]

You're right. In fact, if we trust try ... catch KeyError logic, then the cache doesn't have to have a __contains__ operation defined, and it can be more thread-safe because of the time that elapses between the key check and the extraction.

[nsmith-]

In python's Mapping abc, __contains__ is not abstract because of this very logic.