Add copy and free function annotations for POD types

Plain Old Data (POD) types with or without a representation in the GType
type system can still have a copy and/or a free function. We should
allow annotating these types with their corresponding functions for
copying their data into a new instance, and freeing their data.

From a language bindings perspective, POD types should have a boxed
GType wrapper around them, so they can use the generic GBoxed API to
copy and free instances; from a documentation perspective, though, it'd
be good to have a way to match a structured type, like a struct or a
union, with its copy and free functions.

In order to do that, we add two new header block annotations:

- (copy-func function_name)
- (free-func function_name)

These annotations work exactly like ref-func and unref-func for typed
instances:

    /**
     * GdkRGBA: (copy-func gdk_rgba_copy)
     *   (free-func gdk_rgba_free)
     * @red: ...
     * @green: ...
     * @blue: ...
     * @alpha: ...
     *
     * ...
     */

The function is stored in the GIR data as two new attributes for the
`<record>` and `<union>` elements:

    <record name="RGBA"
            c:type="GdkRGBA"
            copy-function="gdk_rgba_copy"
            free-function="gdk_rgba_free"
            glib:type-name="GdkRGBA"
            glib:get-type="gdk_rgba_get_type"
            c:symbol-prefix="gdk_rgba">

The annotations are not mandatory.

See: #14

giunioninfo.h

@@ -77,6 +77,12 @@ gsize            g_union_info_get_size                 (GIUnionInfo *info);
 GI_AVAILABLE_IN_ALL
 gsize            g_union_info_get_alignment            (GIUnionInfo *info);
 
+GI_AVAILABLE_IN_1_76
+const char *     g_union_info_get_copy_function        (GIUnionInfo *info);
+
+GI_AVAILABLE_IN_1_76
+const char *     g_union_info_get_free_function        (GIUnionInfo *info);
+
 G_END_DECLS