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`
The examples show:
- Building a Signed Pod with different types of values
- Building a MainPod
- Input SignedPod to MainPod
- Input MainPod to MainPod
- Using MainPod or MockMainPod
- Using custom predicates
