core, tests: reduced state copy by N calls

Reduced the amount of state copied that are required by N calls by doing
a balance check prior to any state modifications.

core/vm/environment.go

@@ -36,6 +36,7 @@ type Environment interface {
 	Time() uint64
 	Difficulty() *big.Int
 	GasLimit() *big.Int
+	CanTransfer(from Account, balance *big.Int) bool
 	Transfer(from, to Account, amount *big.Int) error
 	AddLog(*state.Log)
 	AddStructLog(StructLog)