Use const fields in `State` to enforce reuse of memory

[Affie]

We will have to test if performance is improved and the most likely place is with the val field that will be replaced for DFG v1.

[dehann]

I previously tried to use .val field in place, but that ended up being slower. Will be worth retesting once all the type stability and any easy isbitstypes are sorted out.

I'm a bit wary of delaying DFG v1.0 just for this performance optimization -- perhaps there is a way to punt to DFG v1.X if it's not a breaking change?