See review comment for context.

A thought. This API wants to eventually be something where we can call it over and over, picking up where the prior unparse left off in the output. That might be a different API that is slightly different, but in general we need to be able to leave the output in a state such that the next unparse call can continue where the previous left off. Doing this at the bit-position granularity might be too hard, but byte-position ought to be possible.

That's not really about this API, which is symmetric with our parse API. It's just an additional requirement. (The parse API also needs this call-over-and-over capability.)