Callback return value handling.

With this, all NativeCall tests pass on my machine. However, there is
some remaining fragility to deal with (of note, args are not properly
rooted, and we need to panic if an exception is thrown in a callback
or a continuation is taken over a callback boundary; for the time
being, either of these cases will be considered fatal, though they
should be fatal by a good error, not the likely SEGV they lead to
today.)