Add back timsort key function handling (bug#69709)

Add back timsort key function handling (bug#69709)

The original timsort code did provide for a key (accessor) function
along with the necessary storage management, but we dropped it because
our `sort` function didn't need it.

Now it's been put back since it seems that it will come in handy after all.

* src/fns.c (sort_list, sort_vector, Fsort): Pass Qnil as key function
to tim_sort.
* src/sort.c (reverse_slice, sortslice_copy)
(sortslice_copy_incr, sortslice_copy_decr, sortslice_memcpy)
(sortslice_memmove, sortslice_advance): New functions.
(sortslice): New type.
(struct stretch, struct reloc, merge_state)
(binarysort, merge_init, merge_markmem, cleanup_mem)
(merge_register_cleanup, merge_getmem, merge_lo, merge_hi, merge_at)
(found_new_run, reverse_sortslice, resolve_fun, tim_sort):
Merge back previously discarded parts from the upstreams timsort code
that dealt with key functions, and adapt them to fit in.