Design: data type factoring system (and apply to sizes)

[rrnewton]

TBD - this is a placeholder. What's our initial idea on this front? Please fill me in.

[rrnewton]

This is described as "future work" in the ECOOP camera ready.

[rrnewton]

One question that came up in our last meeting was what if we take the extreme position of factoring out all payload fields. That is, the "main" serialized buffer would contain nothing but tags determining the structure of the data.

Here we have a tension where it would be very nice for factoring out size fields to use the same general mechanism as factoring out any fields. But sizes really are specially. We want to use special tricks in encoding the tags of a sized DataCon' vs the original DataCon, and we way want to "remember" the fact the equivalence between those two, allowing the compiler to convert between them where it desires. (For example, automatically "bottoming out" to non-sized constructors as the trees grow small near the leaves.)