add circuit to verify signatures (SignatureVerifyGadget) (#167)

* 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>

src/backends/plonky2/primitives/merkletree.rs

@@ -10,7 +10,6 @@ use std::iter::IntoIterator;
 use crate::backends::counter;
 use crate::backends::plonky2::basetypes::{hash_fields, Hash, Value, EMPTY_HASH, F};
 
-// mod merkletree_circuit;
 pub use super::merkletree_circuit::*;
 
 /// Implements the MerkleTree specified at