Add a new "allocated_slice" argument to _pygi_boxed_new() which specifies
whether its "boxed" pointer was allocated using a slice (by giving its size) or
malloc (by specifying 0), as _pygi_boxed_new cannot determine that itself any
more.

Use this in _pygi_marshal_to_py_interface_struct() for caller-allocated boxed
values, as _caller_alloc() uses _pygi_boxed_alloc() for those (i. e. slices),
which would otherwise leak.

Thanks to Mike Gorse <mgorse@suse.com> for the original patch!

https://bugzilla.gnome.org/show_bug.cgi?id=691501