Decide whether to consider required-with-default field additions as backwards compatible

[sdboyer]

cue-lang/cue#1470 made me realize that i had an erroneous assumption about how cue.Value.Subsume() worked. It's OK, even preferable that there's a simpler definition of it than my erroneous assumption allowed for, but it forces us to make a decision with Thema: should the addition of a new required field that does have a default value be considered a backwards compatible change, and therefore be allowed within a sequence?

My initial inclination is to say that we do, but the implications of this choice are considerable.

(TODO write up the implications re: default values, de/hydration, and assigning semantics to the state space)