Fix SinglyLinkedRing in lists module

- SinglyLinkedRing's prepend was broken
- needed a tail so that prepend can work properly
- now append works as well, so I added it too
- simple testcase added as well

tests/stdlib/tsinglylinkedring.nim

@@ -0,0 +1,29 @@
+discard """
+  output: '''[5]
+[4, 5]
+[3, 4, 5]
+[2, 3, 4, 5]
+[2, 3, 4, 5, 6]
+[2, 3, 4, 5, 6, 7]
+[2, 3, 4, 5, 6, 7, 8]
+[1, 2, 3, 4, 5, 6, 7, 8]'''
+"""
+import lists
+
+var r = initSinglyLinkedRing[int]()
+r.prepend(5)
+echo r
+r.prepend(4)
+echo r
+r.prepend(3)
+echo r
+r.prepend(2)
+echo r
+r.append(6)
+echo r
+r.append(7)
+echo r
+r.append(8)
+echo r
+r.prepend(1)
+echo r