Clean up task interface and add testing. (#34)

* Update c extensions and python tests.

* Updates.

* Documentation fix.

* Move c extensions into common_extension.c

lib/python/common_extension.h

@@ -0,0 +1,35 @@
+#ifndef COMMON_EXTENSION_H
+#define COMMON_EXTENSION_H
+
+#include <Python.h>
+#include "marshal.h"
+#include "structmember.h"
+
+#include "common.h"
+#include "task.h"
+
+extern PyObject *CommonError;
+
+// clang-format off
+typedef struct {
+  PyObject_HEAD
+  object_id object_id;
+} PyObjectID;
+
+typedef struct {
+  PyObject_HEAD
+  task_spec *spec;
+} PyTask;
+// clang-format on
+
+extern PyTypeObject PyObjectIDType;
+
+int PyObjectToUniqueID(PyObject *object, object_id *objectid);
+
+PyObject *PyObjectID_make(object_id object_id);
+
+PyObject *check_simple_value(PyObject *self, PyObject *args);
+
+PyObject *PyTask_make(task_spec *task_spec);
+
+#endif /* COMMON_EXTENSION_H */