Update LibClass.lua

Yet another bugfix for list:insert!
Here is a full test case:
for i = -5, 6 do
  t1 = List{"1", "2", "3", "4", "5"}
  t2 = List{"A", "B", "C"}
  t1:insert(i, t2)
  print(i, t1:concat(";"))
end
for i = -3, 4 do
  t1 = List{"1", "2", "3"}
  t1:insert(i, "A")
  print(i, t1:concat(";"))
end