Update panics by `todo!` and `unimplemented!` macros (#2471)

* Add panic message to `unimplemented!`

So that it is clear why the panic happened upon initial inspection. Also
include a reference to the mempool epic, so that it's easier to find the
issue that tracks the implementation of the missing code.

* Add panic message that references a tracking issue

Make it easy to find the relevant issue if the panic occurs.

* Remove incomplete and currently unnecessary code

The current implementation works, the commented out code was just a
previous improvement idea, which is now tracked by issue #2473.

zebra-chain/src/orchard/note/nullifiers.rs

@@ -21,7 +21,7 @@ use super::super::{
 /// [poseidonhash]: https://zips.z.cash/protocol/nu5.pdf#poseidonhash
 fn poseidon_hash(_x: pallas::Base, _y: pallas::Base) -> pallas::Base {
     // TODO: implement: #2064
-    unimplemented!()
+    unimplemented!("PoseidonHash is not yet implemented (#2064)")
 }
 
 /// Used as part of deriving the _nullifier_ for a Orchard _note_.

zebra-consensus/src/transaction.rs

@@ -43,9 +43,6 @@ mod tests;
 pub struct Verifier<ZS> {
     network: Network,
     script_verifier: script::Verifier<ZS>,
-    // spend_verifier: groth16::Verifier,
-    // output_verifier: groth16::Verifier,
-    // joinsplit_verifier: groth16::Verifier,
 }
 
 impl<ZS> Verifier<ZS>
@@ -53,16 +50,10 @@ where
     ZS: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
     ZS::Future: Send + 'static,
 {
-    // XXX: how should this struct be constructed?
     pub fn new(network: Network, script_verifier: script::Verifier<ZS>) -> Self {
-        // let (spend_verifier, output_verifier, joinsplit_verifier) = todo!();
-
         Self {
             network,
             script_verifier,
-            // spend_verifier,
-            // output_verifier,
-            // joinsplit_verifier,
         }
     }
 }
@@ -153,7 +144,7 @@ where
         };
         if is_mempool {
             // XXX determine exactly which rules apply to mempool transactions
-            unimplemented!();
+            unimplemented!("Zebra does not yet have a mempool (#2309)");
         }
 
         let script_verifier = self.script_verifier.clone();