- Add fits_in_len_bytes assertions for packed stores.
Corrects the assertion formerly on line 888.
- In exclusive mode, don't overwrite packedCtrlSize with the max of the
group; each repeat should know how many bytes it is using, even if
they share the same stream state.
- Ensure that exclusive mode stream state is sized correctly.
In some ring-based models, we know that if the ring is not stale, then
all our bounds should fit within 32-bits. This change makes these
explicitly u32 rather than implicitly narrowing later on.