Remove unnecessary mut in PodSigner trait (#340)

The PodSigner trait was taking `&mut self` in the `sign` method, but the
signer doesn't need mutation in the Shcnorr implementation.  Remove the
`mut`.

Previously the PodProver trait was also taking `&mut self` in the
`prove` method, and we had many tests creating a `mut Prover/mut
MockProver`.  Remove all those `mut`.

Breaking change: `PodSigner` trait method `sign` replaces `&mut self` by
`&self`

examples/signed_pod.rs

@@ -17,7 +17,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     let pk = sk.public_key();
     println!("Public key: {}\n", pk);
 
-    let mut signer = Signer(sk);
+    let signer = Signer(sk);
 
     // Build the signed pod
     let mut builder = SignedPodBuilder::new(&params);
@@ -35,7 +35,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     );
 
     // Sign the pod and verify it
-    let pod = builder.sign(&mut signer)?;
+    let pod = builder.sign(&signer)?;
     pod.verify()?;
 
     println!("{}", pod);