feat: vm.MutableStack wrapper (#31)

* feat: `vm.MutableStack` wrapper

* refactor: use `require.Empty()`

core/vm/stack.libevm.go

@@ -0,0 +1,14 @@
+package vm
+
+import "github.com/holiman/uint256"
+
+// A MutableStack embeds a Stack to expose unexported mutation methods.
+type MutableStack struct {
+	*Stack
+}
+
+// Push pushes a value to the stack.
+func (s MutableStack) Push(d *uint256.Int) { s.Stack.push(d) }
+
+// Pop pops a value from the stack.
+func (s MutableStack) Pop() uint256.Int { return s.Stack.pop() }

core/vm/stack.libevm_test.go

@@ -0,0 +1,27 @@
+package vm_test
+
+import (
+	"testing"
+
+	"github.com/holiman/uint256"
+	"github.com/stretchr/testify/require"
+
+	"github.com/ethereum/go-ethereum/core/vm"
+)
+
+func TestMutableStack(t *testing.T) {
+	s := &vm.Stack{}
+	m := vm.MutableStack{Stack: s}
+
+	push := func(u uint64) uint256.Int {
+		u256 := uint256.NewInt(u)
+		m.Push(u256)
+		return *u256
+	}
+
+	require.Empty(t, s.Data(), "new stack")
+	want := []uint256.Int{push(42), push(314159), push(142857)}
+	require.Equalf(t, want, s.Data(), "after pushing %d values to empty stack", len(want))
+	require.Equal(t, want[len(want)-1], m.Pop(), "popped value")
+	require.Equal(t, want[:len(want)-1], s.Data(), "after popping a single value")
+}