Clean up more code

The *NSYNC linked list API is good enough that it deserves to be part of
the C libray, so this change writes an improved version of it which uses
that offsetof() trick from the Linux Kernel. We vendor all of the *NSYNC
tests in third_party which helped confirm the needed refactoring is safe

This change also deletes more old code that didn't pan out. My goal here
is to work towards a vision where the Cosmopolitan core libraries become
less experimental and more focused on curation. This better reflects the
current level of quality we've managed to achieve.

libc/intrin/dll.c

@@ -0,0 +1,93 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/intrin/dll.h"
+
+/**
+ * @fileoverview Doubly Linked List Library
+ * Based on designs used by Mike Burrows and Linus Torvalds.
+ */
+
+/**
+ * Makes `succ` and its successors come after `elem`.
+ *
+ * It's required that `elem` and `succ` aren't part of the same list.
+ */
+void dll_splice_after(struct Dll *elem, struct Dll *succ) {
+  struct Dll *tmp1, *tmp2;
+  tmp1 = elem->next;
+  tmp2 = succ->prev;
+  elem->next = succ;
+  succ->prev = elem;
+  tmp2->next = tmp1;
+  tmp1->prev = tmp2;
+}
+
+/**
+ * Removes item from doubly-linked list.
+ *
+ * @param list is a doubly-linked list, where `!*list` means empty
+ */
+void dll_remove(struct Dll **list, struct Dll *elem) {
+  if (*list == elem) {
+    if ((*list)->prev == *list) {
+      *list = 0;
+    } else {
+      *list = (*list)->prev;
+    }
+  }
+  elem->next->prev = elem->prev;
+  elem->prev->next = elem->next;
+  elem->next = elem;
+  elem->prev = elem;
+}
+
+/**
+ * Inserts items into list, at the beginning.
+ *
+ * The resulting list will start with `elem`, followed by other items in
+ * `elem`, followed by the items previously in `*list`.
+ *
+ * @param list is a doubly-linked list, where `!*list` means empty
+ * @param elem must not be a member of `list`, or null for no-op
+ */
+void dll_make_first(struct Dll **list, struct Dll *elem) {
+  if (elem) {
+    if (!*list) {
+      *list = elem->prev;
+    } else {
+      dll_splice_after(*list, elem);
+    }
+  }
+}
+
+/**
+ * Inserts items into list, at the end.
+ *
+ * The resulting `*list` will end with `elem`, preceded by the other
+ * items in `elem`, preceded by the items previously in `*list`.
+ *
+ * @param list is a doubly-linked list, where `!*list` means empty
+ * @param elem must not be a member of `list`, or null for no-op
+ */
+void dll_make_last(struct Dll **list, struct Dll *elem) {
+  if (elem) {
+    dll_make_first(list, elem->next);
+    *list = elem;
+  }
+}