Avoid Env#asGuestValue for Truffle::Interop methods

[eregon]

From discussion at #2257 (comment)

For Truffle::Interop.source_location, [String, Integer] seems obvious.

For date/time-related InteropLibrary messages, it's not that clear, but at least we should treat foreign date+time+zone the same as Ruby's Time.

@fniephaus How do you actually use the result of asGuestValue?
It seems not usable at all from #2257 (comment).

[fniephaus]

I must admit I don't understand why asGuestValue doesn't work. Is it because it considers visibility of packages?
Anyway, we have our own wrapper class for exposing host objects in TruffleSqueak, which shamelessly exposes everything possible to guest languages.