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/giunresolvedinfo.h

@@ -0,0 +1,43 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ * GObject introspection: Callable
+ *
+ * Copyright 2023 GNOME Foundation, Inc.
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#pragma once
+
+#if !defined (__GIREPOSITORY_H_INSIDE__) && !defined (GI_COMPILATION)
+#error "Only <girepository.h> can be included directly."
+#endif
+
+#include <girepository/gitypes.h>
+
+G_BEGIN_DECLS
+
+/**
+ * GI_IS_UNRESOLVED_INFO
+ * @info: an info structure
+ *
+ * Checks if @info is a #GIUnresolvedInfo or derived from it.
+ */
+#define GI_IS_UNRESOLVED_INFO(info)					\
+    (gi_base_info_get_info_type ((GIBaseInfo*) info) == GI_INFO_TYPE_UNRESOLVED)
+
+G_END_DECLS