Arbitrary sized Variant

[dsimcha]

I've started looking at implementing arbitrary sized Variant, and
I've hit a point where I need some opinions.  The idea of
arbitrary sized Variant is to heap allocate when things get too
big to be stored inline in the VariantN struct.  However, how
should copying a VariantN struct with the contents stored on the
heap be handled?

If I don't copy the underlying heap space (thus giving reference
semantics), then the semantics of Variant depend in a non-
intuitive way on the size of the item stored in it.  This is
obviously bad.

If I do copy the data, this results in a ridiculous number of
hidden heap allocations.  This is obviously bad.

I guess the available options would be to make a completely
separate Variant type for arbitrary size (annoying, not as
transparent as anyone would like), or to try to implement some
kind of copy-on-write semantics (would require much deeper/more
fundamental changes to the existing code).