compiler/interop: add missing methods to interop.Iterator

Add Next() and Value() to interop.Iterator and corresponding syscalls to
compiler

pkg/compiler/syscall.go

@@ -86,6 +86,8 @@ var syscalls = map[string]map[string]string{
 		"Create": "Neo.Iterator.Create",
 		"Key":    "Neo.Iterator.Key",
 		"Keys":   "Neo.Iterator.Keys",
+		"Next":   "Neo.Iterator.Next",
+		"Value":  "Neo.Iterator.Value",
 		"Values": "Neo.Iterator.Values",
 	},
 }

pkg/interop/iterator/iterator.go

@@ -22,6 +22,17 @@ func Keys(it Iterator) []interface{} {
 	return nil
 }
 
+// Next advances the iterator, return true if it is was successful
+// and false otherwise.
+func Next(it Iterator) bool {
+	return true
+}
+
+// Value returns the current iterator value.
+func Value(it Iterator) interface{} {
+	return nil
+}
+
 // Values returns the iterator values.
 func Values(it Iterator) []interface{} {
 	return nil