[Vala] malloc/free CCode attributes

[Arc Riley <arcriley ubuntu com>]

It would be extremely helpful in developing bindings for other object systems if the glib's memory management could be overridden, ie;

  [Compact]

  [CCode (lower_case_cprefix = "PyObject_",
          ref_function = "Py_IncRef",

          ref_function_void = true,
          unref_function = "Py_DecRef",

          alloc_function = "PyObject_Malloc",
          free_function = "PyObject_Free")]
  public class Object {

...

Thoughts?