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

client/src/thin_client.rs

@@ -336,7 +336,7 @@ impl SyncClient for ThinClient {
         keypair: &Keypair,
         instruction: Instruction,
     ) -> TransportResult<Signature> {
-        let message = Message::new(&[instruction]);
+        let message = Message::new(&[instruction], Some(&keypair.pubkey()));
         self.send_message(&[keypair], message)
     }
 
@@ -574,7 +574,7 @@ impl AsyncClient for ThinClient {
         instruction: Instruction,
         recent_blockhash: Hash,
     ) -> TransportResult<Signature> {
-        let message = Message::new(&[instruction]);
+        let message = Message::new(&[instruction], Some(&keypair.pubkey()));
         self.async_send_message(&[keypair], message, recent_blockhash)
     }
     fn async_transfer(