budget as separate contract and system call contract (#1189)

* budget and system contracts and verification

* contract check_id methods
* system call contract
* verify contract execution rules
* move system into its own file
* allocate before transfer for budget
* store error in budget context
* budget contract and tests without bank
* moved budget of of bank

src/system_contract.rs

@@ -0,0 +1,80 @@
+//! system smart contract
+
+use bank::Account;
+use bincode::deserialize;
+use signature::Pubkey;
+use transaction::Transaction;
+
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub enum SystemContract {
+    /// Create a new account
+    /// * Transaction::keys[0] - source
+    /// * Transaction::keys[1] - new account key
+    /// * tokens - number of tokens to transfer to the new account
+    /// * space - memory to allocate if greater then zero
+    /// * contract - the contract id of the new account
+    CreateAccount {
+        tokens: i64,
+        space: u64,
+        contract_id: Option<Pubkey>,
+    },
+    /// Assign account to a contract
+    /// * Transaction::keys[0] - account to assign
+    Assign { contract_id: Pubkey },
+    /// Move tokens
+    /// * Transaction::keys[0] - source
+    /// * Transaction::keys[1] - destination
+    Move { tokens: i64 },
+}
+
+pub const SYSTEM_CONTRACT_ID: [u8; 32] = [0u8; 32];
+
+impl SystemContract {
+    pub fn check_id(contract_id: &Pubkey) -> bool {
+        contract_id.as_ref() == SYSTEM_CONTRACT_ID
+    }
+
+    pub fn id() -> Pubkey {
+        Pubkey::new(&SYSTEM_CONTRACT_ID)
+    }
+    pub fn get_balance(account: &Account) -> i64 {
+        account.tokens
+    }
+    pub fn process_transaction(tx: &Transaction, accounts: &mut [Account]) {
+        let syscall: SystemContract = deserialize(&tx.userdata).unwrap();
+        match syscall {
+            SystemContract::CreateAccount {
+                tokens,
+                space,
+                contract_id,
+            } => {
+                if !Self::check_id(&accounts[1].contract_id) {
+                    return;
+                }
+                if !Self::check_id(&accounts[0].contract_id) {
+                    return;
+                }
+                if space > 0 && !accounts[1].userdata.is_empty() {
+                    return;
+                }
+                accounts[0].tokens -= tokens;
+                accounts[1].tokens += tokens;
+                if let Some(id) = contract_id {
+                    accounts[1].contract_id = id;
+                }
+                accounts[1].userdata = vec![0; space as usize];
+            }
+            SystemContract::Assign { contract_id } => {
+                if !Self::check_id(&accounts[0].contract_id) {
+                    return;
+                }
+                accounts[0].contract_id = contract_id;
+            }
+            SystemContract::Move { tokens } => {
+                //bank should be verifying correctness
+                accounts[0].tokens -= tokens;
+                accounts[1].tokens += tokens;
+            }
+        }
+    }
+}