make std/lists usable with ptr instead of ref

[timotheecour]

just to illustrate that it's possible with a small diff, see tests

note

it'd be better if something like this worked, which depends on nim-lang/RFCs#85 (default generic params):

type
  SinglyLinkedList*[T; isPtr: static bool = false] = object
    when isPtr:
      head*: SinglyLinkedNode[T, true]
      tail*: SinglyLinkedNode[T, true]
    else:
      head*: <//>(SinglyLinkedNode[T])
      tail* {.cursor.}: SinglyLinkedNode[T]