Discard killed buffers from deleted window and frame objects.

Discard killed buffers from deleted window and frame objects.
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.