Replace bincode with borsh (#452)

* Add SignalsConfig to chain_spec

* Correct multiexp feature flagging for rand_core std

* Remove bincode for borsh

Replaces a non-canonical encoding with a canonical encoding which additionally
should be faster.

Also fixes an issue where we used bincode in transcripts where it cannot be
trusted.

This ended up fixing a myriad of other bugs observed, unfortunately.
Accordingly, it either has to be merged or the bug fixes from it must be ported
to a new PR.

* Make serde optional, minimize usage

* Make borsh an optional dependency of substrate/ crates

* Remove unused dependencies

* Use [u8; 64] where possible in the processor messages

* Correct borsh feature flagging

processor/src/batch_signer.rs

@@ -191,7 +191,7 @@ impl<D: Db> BatchSigner<D> {
 
       let (machine, preprocess) = machine.preprocess(&mut OsRng);
       machines.push(machine);
-      serialized_preprocesses.push(preprocess.serialize());
+      serialized_preprocesses.push(preprocess.serialize().try_into().unwrap());
       preprocesses.push(preprocess);
     }
     self.preprocessing.insert(id, (machines, preprocesses));