interop: move into pkg/interop, replace pkg/vm/api

neo-storm has developed more wrappers for syscall APIs, so they can and should
be used as a drop-in replacement for pkg/vm/api. Moving it out of vm, as it's
not exactly related to the VM itself.

pkg/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-go 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
+}