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

faucet/tests/local-faucet.rs

@@ -16,7 +16,7 @@ fn test_local_faucet() {
     let lamports = 50;
     let blockhash = Hash::new(&to.as_ref());
     let create_instruction = system_instruction::transfer(&keypair.pubkey(), &to, lamports);
-    let message = Message::new(&[create_instruction]);
+    let message = Message::new(&[create_instruction], Some(&keypair.pubkey()));
     let expected_tx = Transaction::new(&[&keypair], message, blockhash);
 
     let (sender, receiver) = channel();