dbc: convert method information for DBC proofs from const to fn

dbc/src/opret/mod.rs

@@ -72,7 +72,8 @@ impl StrictDeserialize for OpretProof {}
 
 impl Proof<Method> for OpretProof {
     type Error = EmbedVerifyError<OpretError>;
-    const METHOD: Method = Method::OpretFirst;
+
+    fn method(&self) -> Method { Method::OpretFirst }
 
     fn verify(&self, msg: &Commitment, tx: &Tx) -> Result<(), EmbedVerifyError<OpretError>> {
         tx.verify(msg, self)

dbc/src/proof.rs

@@ -91,8 +91,8 @@ pub trait Proof<M: DbcMethod = Method>:
     /// Verification error.
     type Error: Error;
 
-    /// Returns DBC method used by the proof.
-    const METHOD: M;
+    /// Returns a single-use seal closing method used by the DBC proof.
+    fn method(&self) -> M;
 
     /// Verifies DBC proof against the provided transaction.
     fn verify(&self, msg: &mpc::Commitment, tx: &Tx) -> Result<(), Self::Error>;

dbc/src/tapret/mod.rs

@@ -377,7 +377,8 @@ impl TapretProof {
 
 impl Proof<Method> for TapretProof {
     type Error = ConvolveVerifyError;
-    const METHOD: Method = Method::TapretFirst;
+
+    fn method(&self) -> Method { Method::TapretFirst }
 
     fn verify(&self, msg: &Commitment, tx: &Tx) -> Result<(), ConvolveVerifyError> {
         ConvolveCommitProof::<_, Tx, _>::verify(self, msg, tx)