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`

src/middleware/mod.rs

@@ -874,7 +874,7 @@ dyn_clone::clone_trait_object!(RecursivePod);
 
 pub trait PodSigner {
     fn sign(
-        &mut self,
+        &self,
         params: &Params,
         kvs: &HashMap<Key, Value>,
     ) -> Result<Box<dyn Pod>, BackendError>;