ruby: new notmuch_rb_object_destroy() helper

The struct used to store the types (rb_data_type_t) contains a "data"
field where we can store whatever we want. I use that field to store a
pointer to the corresponding destroy function. For example
notmuch_rb_database_type contains a pointer to notmuch_database_destroy.

I cast that pointer as a notmuch_status_t (func*)(void *) and call
that function passing the internal object (e.g. notmuch_database_t).

Using the rb_data_type_t data we can call the correct notmuch destroy
function.

Therefore this:

  ret = ((notmuch_status_t (*)(void *)) type->data) (nm_object);

Is effectively the same as this:

  ret = notmuch_database_destroy (database);

The advantage of doing it this way is that much less code is necesary
since each rb_data_type_t has the corresponding destroy function stored
in it.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>