Fix List.split

This commit makes List.split behave a bit more sane and elegantly.
Currently split has the following behaviour:

  0: split (== ':") "foo"   -> ["foo"]
  1: split (== ':") ":foo"  -> ["", "foo"]
  2: split (== ':") "foo:"  -> ["foo"]     (!!)

  3: split (== ':') ":foo:foo"   -> ["", "foo", "foo"]
  4: split (== ':') ":foo"       -> ["", "foo"]
  5: split (== ':') ""           -> []     (!!)

Cases 3 and 5 are the problematic ones. I would think they would result
in ["foo", ""] and [""] respectively. This patch gives split this
behaviour, it also gives split the nice property that the length of
(split ':' str) is (1 + the number of colons in str)

I've looked at the uses of split in the prelude and none of them seem to
be effected by this change.