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

programs/exchange_api/src/lib.rs

@@ -11,3 +11,22 @@ pub const EXCHANGE_PROGRAM_ID: [u8; 32] = [
 ];
 
 solana_sdk::solana_program_id!(EXCHANGE_PROGRAM_ID);
+
+pub const EXCHANGE_FAUCET_ID: [u8; 32] = [
+    3, 147, 111, 103, 210, 47, 23, 11, 176, 29, 147, 89, 237, 155, 21, 62, 107, 105, 157, 1, 98,
+    204, 206, 211, 54, 212, 79, 15, 160, 0, 0, 0,
+];
+
+pub fn faucet_id() -> solana_sdk::pubkey::Pubkey {
+    solana_sdk::pubkey::Pubkey::new(&EXCHANGE_FAUCET_ID)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn exchange_faucet_id() {
+        let ids = [("ExchangeFaucet11111111111111111111111111111", faucet_id())];
+        assert!(ids.iter().all(|(name, id)| *name == id.to_string()));
+    }
+}