O(1) concatenation of singly- and doubly linked lists. (#16362)

* O(1) concatenation of singly- and doubly linked lists.

There is also new `toSinglyLinkedList` and `toDoublyLinkedList`
functions for conversion from `openArray`s, similarly
to `toHashSet` or `toTable`.

* Add `sequtils` import to runnable examples with `toSeq`.

* Added missing call to runnable examples.

* Add .since annotation, changelog, and tests.

* Rename `lists.concat` as an overload to `lists.append`.

* Renamed `append` to `add` in lists.

* Remove faulty `add` for `DoublyLinkedList`s.

* Improved tests for lists.

* `lists.add` moves the second list; added `lists.copy` for shallow copy.

* More tests for `lists.add` and `lists.copy`.

* More compact tests for lists with templates.

* List concatenation with copying (`add`) and moving (tentatively `addMove`)

* Renamed `addMove` to `addMoved`; renamed arguments according to the style guide.

* Added extended example to `lists.copy`.

* Corrected .since annotations to 1.6

* Add .since annotation, changelog, and tests.

* Rename `lists.concat` as an overload to `lists.append`.

* Renamed `append` to `add` in lists.

* Remove faulty `add` for `DoublyLinkedList`s.

* `lists.add` moves the second list; added `lists.copy` for shallow copy.

* More tests for `lists.add` and `lists.copy`.

* List concatenation with copying (`add`) and moving (tentatively `addMove`)

* Renamed `addMove` to `addMoved`; renamed arguments according to the style guide.

* Since declarations changed to (1,5,1).

* Add .since annotation, changelog, and tests.

* Rename `lists.concat` as an overload to `lists.append`.

* Renamed `append` to `add` in lists.

* Remove faulty `add` for `DoublyLinkedList`s.

* `lists.add` moves the second list; added `lists.copy` for shallow copy.

* More tests for `lists.add` and `lists.copy`.

* List concatenation with copying (`add`) and moving (tentatively `addMove`)

* Renamed `addMove` to `addMoved`; renamed arguments according to the style guide.

* Changelog update.

* Fix rebasing errors.

* Self-adding with `lists.addMove` results in a cycle now.
Added some extra tests.

tests/stdlib/tlists.nim

@@ -2,7 +2,7 @@ discard """
   targets: "c js"
 """
 
-import lists
+import lists, sequtils, std/enumerate, std/sugar
 
 const
   data = [1, 2, 3, 4, 5, 6]
@@ -81,3 +81,70 @@ block tlistsToString:
     l.append('2')
     l.append('3')
     doAssert $l == """['1', '2', '3']"""
+
+template testCommon(initList, toList) =
+
+  block: # toSinglyLinkedList, toDoublyLinkedList
+    let l = seq[int].default
+    doAssert l.toList.toSeq == []
+    doAssert [1].toList.toSeq == [1]
+    doAssert [1, 2, 3].toList.toSeq == [1, 2, 3]
+
+  block copy:
+    doAssert array[0, int].default.toList.copy.toSeq == []
+    doAssert [1].toList.copy.toSeq == [1]
+    doAssert [1, 2].toList.copy.toSeq == [1, 2]
+    doAssert [1, 2, 3].toList.copy.toSeq == [1, 2, 3]
+    type Foo = ref object
+      x: int
+    var f0 = Foo(x: 0)
+    let f1 = Foo(x: 1)
+    var a = [f0].toList
+    var b = a.copy
+    b.append f1
+    doAssert a.toSeq == [f0]
+    doAssert b.toSeq == [f0, f1]
+    f0.x = 42
+    assert a.head.value.x == 42
+    assert b.head.value.x == 42
+
+  block: # add, addMoved
+    block:
+      var
+        l0 = initList[int]()
+        l1 = [1].toList
+        l2 = [2, 3].toList
+        l3 = [4, 5, 6].toList
+      l0.add l3
+      l1.add l3
+      l2.addMoved l3
+      doAssert l0.toSeq == [4, 5, 6]
+      doAssert l1.toSeq == [1, 4, 5, 6]
+      doAssert l2.toSeq == [2, 3, 4, 5, 6]
+      doAssert l3.toSeq == []
+      l2.add l3 # re-adding l3 that was destroyed is now a no-op
+      doAssert l2.toSeq == [2, 3, 4, 5, 6]
+      doAssert l3.toSeq == []
+    block:
+      var
+        l0 = initList[int]()
+        l1 = [1].toList
+        l2 = [2, 3].toList
+        l3 = [4, 5, 6].toList
+      l3.addMoved l0
+      l2.addMoved l1
+      doAssert l3.toSeq == [4, 5, 6]
+      doAssert l2.toSeq == [2, 3, 1]
+      l3.add l0
+      doAssert l3.toSeq == [4, 5, 6]
+    block:
+      var c = [0, 1].toList
+      c.addMoved c
+      let s = collect:
+        for i, ci in enumerate(c):
+          if i == 6: break
+          ci
+      doAssert s == [0, 1, 0, 1, 0, 1]
+
+testCommon initSinglyLinkedList, toSinglyLinkedList
+testCommon initDoublyLinkedList, toDoublyLinkedList