make unsnoc handle infinite lists

[mauke]

Before:

ghci> take 10 . fst <$> unsnoc [1..]
Just *** Exception: stack overflow

After:

ghci> take 10 . fst <$> unsnoc [1..]
Just [1,2,3,4,5,6,7,8,9,10]

[mauke]

... see also haskell/core-libraries-committee#165:

It's important to use a lazy pattern match ~(a, b) in the definition of unsnoc above. Otherwise the function becomes non-productive on infinite lists and (probably more importantly) is prone to stack overflow.

... which is exactly the issue I stumbled over.