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/util/util.go

@@ -0,0 +1,12 @@
+package util
+
+// FromAddress is an utility function that converts an NEO address to its hash.
+func FromAddress(address string) []byte {
+	return nil
+}
+
+// Equals compares a with b and will return true whether a and b
+// are equal.
+func Equals(a, b interface{}) bool {
+	return false
+}