Specialize transaction assets to i64

Proof-of-history is generic, but now that we're using it entirely
for tokens, we can specialize the type and start doing more interesting
things than just Eq and Serialize operations.

src/accountant_stub.rs

@@ -26,7 +26,7 @@ impl AccountantStub {
         }
     }
 
-    pub fn transfer_signed(&self, tr: Transaction<i64>) -> io::Result<usize> {
+    pub fn transfer_signed(&self, tr: Transaction) -> io::Result<usize> {
         let req = Request::Transaction(tr);
         let data = serialize(&req).unwrap();
         self.socket.send_to(&data, &self.addr)