Response to usage of unwrap in non-test code

This commit replaces all usage of `unwrap` with `expect` within
`networks/monero`, clarifying why the panic risked is unreachable. This commit
also replaces some uses of `unwrap` with solutions which are guaranteed not to
fail.

Notably, compilation on 128-bit systems is prevented, ensuring
`u64::try_from(usize::MAX)` will never panic at runtime.

Slight breaking changes are additionally included as necessary to massage out
some avoidable panics.

networks/monero/wallet/src/send/multisig.rs

@@ -251,7 +251,7 @@ impl SignMachine<Transaction> for TransactionSignMachine {
     }
 
     let tx = tx.transaction_without_signatures();
-    let msg = tx.signature_hash().unwrap();
+    let msg = tx.signature_hash().expect("signing a transaction which isn't signed?");
 
     // Iterate over each CLSAG calling sign
     let mut shares = Vec::with_capacity(to_sign.len());