core/state: add BalanceChangeReason to state db and state object (#74)

* core/state: add BalanceChangeReason to state object

* core/state: add BalanceChangeReason to StateDB.AddBalance

* core/state: add BalanceChangeReason to StateDB.SubBalance

* core/state: add BalanceChangeReason to StateDB.SetBalance

Author: sonhv0212

core/vm/evm.go

@@ -325,7 +325,7 @@ func (evm *EVM) CallCode(caller ContractRef, addr common.Address, input []byte,
 	if !evm.Context.CanTransfer(evm.StateDB, caller.Address(), value) {
 		return nil, gas, ErrInsufficientBalance
 	}
-	var snapshot = evm.StateDB.Snapshot()
+	snapshot := evm.StateDB.Snapshot()
 
 	// It is allowed to call precompiles, even via delegatecall
 	if p, isPrecompile := evm.precompile(caller, addr); isPrecompile {
@@ -376,7 +376,7 @@ func (evm *EVM) DelegateCall(caller ContractRef, addr common.Address, input []by
 	if evm.depth > int(params.CallCreateDepth) {
 		return nil, gas, ErrDepth
 	}
-	var snapshot = evm.StateDB.Snapshot()
+	snapshot := evm.StateDB.Snapshot()
 
 	// It is allowed to call precompiles, even via delegatecall
 	if p, isPrecompile := evm.precompile(caller, addr); isPrecompile {
@@ -425,7 +425,7 @@ func (evm *EVM) StaticCall(caller ContractRef, addr common.Address, input []byte
 	// after all empty accounts were deleted, so this is not required. However, if we omit this,
 	// then certain tests start failing; stRevertTest/RevertPrecompiledTouchExactOOG.json.
 	// We could change this, but for now it's left for legacy reasons
-	var snapshot = evm.StateDB.Snapshot()
+	snapshot := evm.StateDB.Snapshot()
 
 	// We do an AddBalance of zero here, just in order to trigger a touch.
 	// This doesn't matter on Mainnet, where all empties are gone at the time of Byzantium,