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/enumerator/enumerator.go

@@ -0,0 +1,29 @@
+package enumerator
+
+// Package enumerator provides function signatures that can be used inside
+// smart contracts that are written in the neo-go framework.
+
+// TODO: Check enumerator use cases and add them to the examples folder.
+
+// Enumerator stubs a NEO enumerator type.
+type Enumerator struct{}
+
+// Create creates a new enumerator from the given items.
+func Create(items []interface{}) Enumerator {
+	return Enumerator{}
+}
+
+// Next returns the next item in the iteration.
+func Next(e Enumerator) interface{} {
+	return nil
+}
+
+// Value returns the enumerator value.
+func Value(e Enumerator) interface{} {
+	return nil
+}
+
+// Concat concats the 2 given enumerators.
+func Concat(a, b Enumerator) Enumerator {
+	return Enumerator{}
+}