Remove `all_buffers` and the associated `next` field of buffers

Remove `all_buffers` and the associated `next` field of buffers

* src/alloc.c (enum mem_type): Remove MEM_TYPE_BUFFER.
(allocate_buffer): Allocate like any other pseudovector.
Don't register on `all_buffers` any more.
(live_buffer_holding, live_buffer_p): Delete functions.
(mark_maybe_object, valid_lisp_object_p): Don't pay attention to
MEM_TYPE_BUFFER any more.
(garbage_collect): Only compact the live buffers.
(mark_buffer): Mark the undo_list of dead buffers here.
(mark_object): Buffers are normal pseudovectors now.
(sweep_buffers): Don't do the actual sweep here, just cleanup the
markers and only for live buffers.

* src/buffer.c (all_buffers): Remove variable.
(Fkill_buffer): Don't check indirect dead buffers.
Set the undo_list before we remove ourselves from the list of live buffers.
(Fbuffer_swap_text, Fset_buffer_multibyte): Don't check indirect dead
buffers.
(init_buffer_once): Don't set `all_buffers`.
(init_buffer): Don't map new memory for dead buffers.

* src/buffer.h (struct buffer): Remove `next` field.
(FOR_EACH_BUFFER): Remove macro.

* src/pdumper.c (dump_buffer): Don't dump the `next` field.