note some simple properties.

should we specify properties for take, drop, etc as well? because those
are defined on seq it doesn't seem "sensible", but could probably help
as well.

hs/DataStructures.hs

@@ -156,3 +156,9 @@ instance Seq BankersQueue where
-- examples
sl = fromList [1..10] :: List Integer
bq = fromList [1..10] :: BankersQueue Integer
-- properties
-- rest nil == nil
-- (reverse . reverse) s == s
-- first [s] == last [s]