This avoids:
* triple-checking for the GType when type-checking is enabled (see #597260)
* Avoids going through an expensive no-argument checking which landed in
glib-2.22
* Avoids going through 2 extrac functions (g_object_new -> g_object_new_valist)
Fix an off-by-one error in the size guard for unpack_element, and
improve various debug statements in the failure paths.
Also, swap some g_new0 to g_malloc0 for the fun of it.
