core/vm: reduced big int allocations

Reduced big int allocation by making stack items modifiable. Instead of
adding items such as `common.Big0` to the stack, `new(big.Int)` is
added instead. One must expect that any item that is added to the stack
might change.

core/vm/stack.go

@@ -21,14 +21,17 @@ import (
 	"math/big"
 )
 
-func newstack() *stack {
-	return &stack{}
-}
-
+// stack is an object for basic stack operations. Items popped to the stack are
+// expected to be changed and modified. stack does not take care of adding newly
+// initialised objects.
 type stack struct {
 	data []*big.Int
 }
 
+func newstack() *stack {
+	return &stack{}
+}
+
 func (st *stack) Data() []*big.Int {
 	return st.data
 }