Consider changing Enum's cache to store arrays of the underlying type

[stephentoub]

#44355 (comment)

It may work better to switch the cached values type to be the underlying enum type. It would eliminate the conversions to UInt64 (we would have to convert or switch over the underlying type instead), and it would also make the cache 2x smaller for the typical case.

[stephentoub]

This has now been implemented.