* 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
* add boolean selector to the MerkleProofGadget, to allow skipping proof verifications when all the slots are not used (eg. in the SignedPod circuit)
* move existing signedpod's circuits draft to its own file
* implement SignedPodVerify circuit