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

core/src/chacha.rs

@@ -154,7 +154,7 @@ mod tests {
         hasher.hash(&buf[..size]);
 
         //  golden needs to be updated if blob stuff changes....
-        let golden: Hash = "5Pz5KQyNht2nqkJhVd8F9zTFxzoDvbQSzaxQbtCPiyCo"
+        let golden: Hash = "9xb2Asf7UK5G8WqPwsvzo5xwLi4dixBSDiYKCtYRikA"
             .parse()
             .unwrap();