vm: allow to push integer-like items to stack

Perform automatic conversion of non-standard integer types to
int64 if possible.

Closes #776.

pkg/vm/stack_item.go

@@ -87,6 +87,11 @@ func makeStackItem(v interface{}) StackItem {
 		}
 		return makeStackItem(a)
 	default:
+		i64T := reflect.TypeOf(int64(0))
+		if reflect.TypeOf(val).ConvertibleTo(i64T) {
+			i64Val := reflect.ValueOf(val).Convert(i64T).Interface()
+			return makeStackItem(i64Val)
+		}
 		panic(
 			fmt.Sprintf(
 				"invalid stack item type: %v (%v)",

pkg/vm/stack_test.go

@@ -1,9 +1,11 @@
 package vm
 
 import (
+	"math/big"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestPushElement(t *testing.T) {
@@ -20,6 +22,20 @@ func TestPushElement(t *testing.T) {
 	}
 }
 
+func TestStack_PushVal(t *testing.T) {
+	type (
+		i32      int32
+		testByte uint8
+	)
+
+	s := NewStack("test")
+	require.NotPanics(t, func() { s.PushVal(i32(123)) })
+	require.NotPanics(t, func() { s.PushVal(testByte(42)) })
+	require.Equal(t, 2, s.Len())
+	require.Equal(t, big.NewInt(42), s.Pop().Value())
+	require.Equal(t, big.NewInt(123), s.Pop().Value())
+}
+
 func TestPopElement(t *testing.T) {
 	var (
 		s     = NewStack("test")