Transaction format changes toward Credit-Only accounts (#4386)

* Add num_readonly_accounts slice

* Impl programs in account_keys

* Emulate current account-loading functionality using program-account_keys (breaks exchange_program_api tests)

* Fix test

* Add temporary exchange faucet id

* Update chacha golden

* Split num_credit_only_accounts into separate fields

* Improve readability

* Move message field constants into Message

* Add MessageHeader struct and fixup comments

sdk/src/fee_calculator.rs

@@ -14,7 +14,7 @@ impl FeeCalculator {
     }
 
     pub fn calculate_fee(&self, message: &Message) -> u64 {
-        self.lamports_per_signature * u64::from(message.num_required_signatures)
+        self.lamports_per_signature * u64::from(message.header.num_required_signatures)
     }
 }