Add an overload of `ArrayCopy` that takes `UnknownArrayHandle`s and
copies them for (almost) any `ArrayHandle` type.

The code uses `CastAndCallWithExtractedArray` to reduce the total number
of copy conditions to about 100, which are all precompiled into the
library. On a debug build on my mac, this creates a .o file of 21MB.
That's not great, but not terrible. Hopefully, this can be used to
consolidate copy implementations elsewhere.