Deferred serialization / deserialization holes

[mgsloan]

packer has a really cool idea where during serialization you can create known-sized holes, and later fill them. I haven't really considered the details, but it would be cool to generalize this to deserialization as well. A hole in deserialization would essentially be explicit decoding laziness. It'd probably use a data frame much like the streaming support.

Another thought on this is that for serialization it'd be good to be able to run some assertion function to check that all the holes have been filled.