Draft of iterator and enumerator (CityOfZion/neo-storm#26)

* Draft of iterator and enumerator

* Added iterator API to the syscall mapping

* Added draft of the enumerator.go file

* Added enumerator interop API.

* Updated the changelog

Imported from CityOfZion/neo-storm (156093318b8612e810965bb1ea26e1babfb46cdd).

interop/iterator/iterator.go

@@ -0,0 +1,28 @@
+package iterator
+
+// Package iterator provides function signatures that can be used inside
+// smart contracts that are written in the neo-storm framework.
+
+// Iterator stubs a NEO iterator object type.
+type Iterator struct{}
+
+// Create creates an iterator from the given items.
+func Create(items []interface{}) Iterator {
+	return Iterator{}
+}
+
+// TODO: Better description for this.
+// Key returns the iterator key.
+func Key(it Iterator) interface{} {
+	return nil
+}
+
+// Keys returns the iterator keys.
+func Keys(it Iterator) []interface{} {
+	return nil
+}
+
+// Values returns the iterator values.
+func Values(it Iterator) []interface{} {
+	return nil
+}