feat: add guards to detect modifying non-natuve element values

[ivokub]

Yes, it is not perfect, but imo works well enough. I have some ideas on how to make it more robust. Namely we can keep some DB of "element pointer -> hash of limbs" and if we see during some non-native operation that there is a conflict then we can panic.

The problem with using values was that then I cannot re-use the element-as-polynomial evaluation value when same element is used in multiple muls. And this really impacted some non-native code. I could possibly try having some additional DB "hash of limbs -> stored evaluation", but dunno if this would be any better than the current approach + integrity check in prev paragraph.

Originally posted by @ivokub in #749 (comment)