Remove fee-payer guesswork from Message and Transaction (#10776)

* Make Message::new_with_payer the default constructor

* Remove Transaction::new_[un]signed_instructions

These guess the fee-payer instead of stating it explicitly

programs/exchange/src/exchange_processor.rs

@@ -604,7 +604,10 @@ mod test {
         );
 
         client
-            .send_message(&[owner, &new], Message::new(&[instruction]))
+            .send_message(
+                &[owner, &new],
+                Message::new(&[instruction], Some(&owner.pubkey())),
+            )
             .unwrap_or_else(|_| panic!("{}:{}", line!(), file!()));
         new.pubkey()
     }