girepository: Port GIBaseInfo to GTypeInstance

This adds more type safety to libgirepository, and allows
differentiating the `GIBaseInfo` derived types using the type system.

Two new derived types had to be added (previously they were just a
collection of helper methods which worked directly on a `GIBaseInfo` and
didn’t check types): `GICallbackInfo` and `GIUnresolvedInfo`.

Further cleanups and refactoring might be needed on this, but the core
of libgirepository now uses `GTypeInstance` and appears to still work
(it’s difficult to be entirely sure because there are no unit tests
yet).

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3155

girepository/gistructinfo.c

@@ -29,6 +29,7 @@
 #include <glib.h>
 
 #include <girepository/girepository.h>
+#include "gibaseinfo-private.h"
 #include "girepository-private.h"
 #include "gitypelib-internal.h"
 #include "gistructinfo.h"
@@ -338,3 +339,12 @@ gi_struct_info_get_free_function (GIStructInfo *info)
 
   return NULL;
 }
+
+void
+gi_struct_info_class_init (gpointer g_class,
+                           gpointer class_data)
+{
+  GIBaseInfoClass *info_class = g_class;
+
+  info_class->info_type = GI_INFO_TYPE_STRUCT;
+}