PL/Python: Add cursor and execute methods to plan object

Instead of

    plan = plpy.prepare(...)
    res = plpy.execute(plan, ...)

you can now write

    plan = plpy.prepare(...)
    res = plan.execute(...)

or even

    res = plpy.prepare(...).execute(...)

and similarly for the cursor() method.

This is more in object oriented style, and makes the hybrid nature of
the existing execute() function less confusing.

Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>

src/pl/plpython/plpy_cursorobject.h

@@ -19,5 +19,6 @@ typedef struct PLyCursorObject
 
 extern void PLy_cursor_init_type(void);
 extern PyObject *PLy_cursor(PyObject *self, PyObject *args);
+extern PyObject *PLy_cursor_plan(PyObject *ob, PyObject *args);
 
 #endif   /* PLPY_CURSOROBJECT_H */