renamed '=' to '=copy' [backport:1.2] (#15585)

* Assign hook name changed to `=copy`
* Adapt destructors.rst
* [nobackport] Duplicate tests for =copy hook
* Fix tests
* added a changelog entry

Co-authored-by: Clyybber <darkmine956@gmail.com>

tests/arc/tcomputedgotocopy.nim

@@ -0,0 +1,41 @@
+discard """
+  cmd: '''nim c --newruntime $file'''
+  output: '''2
+2'''
+"""
+
+type
+  ObjWithDestructor = object
+    a: int
+proc `=destroy`(self: var ObjWithDestructor) =
+  echo "destroyed"
+
+proc `=copy`(self: var ObjWithDestructor, other: ObjWithDestructor) =
+  echo "copied"
+
+proc test(a: range[0..1], arg: ObjWithDestructor) =
+  var iteration = 0
+  while true:
+    {.computedGoto.}
+
+    let
+      b = int(a) * 2
+      c = a
+      d = arg
+      e = arg
+
+    discard c
+    discard d
+    discard e
+
+    inc iteration
+
+    case a
+    of 0:
+      assert false
+    of 1:
+      echo b
+      if iteration == 2:
+        break
+
+test(1, ObjWithDestructor())