This is a solution for #46 that uses RTTI to allow get_user_data to work
and throw an error when used in an archive that doesn't actually have user
data. Unfortunately this is a run-time check and uses a dummy virtual function
that is never actually called (so the overhead of this will be very low, a few bytes
for the vtable, no runtime cost).
Another solution I'm going to play around with involves re-arranging some templates and
typedefs.
