* unify fe/be NativeOp and NativePred
* remove Origin in favour of PodId
* Combine string and hash in Key
* use middleware::AnchoredKey in frontend
* merge frontend/middleware types
* refactor custom predicates
* clean up a bit
* fix middleware custom tests
* clean up
* clean up 2
* add acronyms in typos list
* implement circuit to verify signature (proof-based signature), ie. a 1-level recursion verification
* as agreed in the call, rename Gate -> Gadget when it's not a 'gate'
* make SignatureVerifyGadget conditional on the selector input
* small naming polish
* sigverifygadget: add s computation in-circuit, connect pk,msg,s to internalproof's public_inputs
* optimize signature verify
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Co-authored-by: Eduard S. <eduardsanou@posteo.net>
