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This reduces an amount of references to killed buffers and
helps GC to reclaim them faster.
* alloc.c (discard_killed_buffers): New function.
(mark_object): Use it for deleted windows and frames.
(mark_object): If symbol's value is set up for a killed buffer
or deleted frame, restore it's global binding.
* data.c (swap_in_global_binding): Add GC notice.
(swap_in_symval_forwarding): Use convenient set_blv_where.
* window.c (wset_next_buffers, wset_prev_buffers): Move ...
* window.h: ... to here.