Fix partiality in 'cycle'

[chshersh]

To my big surprise, the cycle function from base is partial: it fails on empty lists

ghci> :t cycle
cycle :: [a] -> [a]
ghci> cycle []
*** Exception: Prelude.cycle: empty list

It's a bit problematic, since we don't want to expose partial functions. I see the following options:

1. Don't reexport cycle.
2. Change its type to NonEmpty a -> NonEmpty a.
3. Reimplement manually by returning empty list when empty list is given.

@vrom911, what do you think?

[vrom911]

I guess the less painful way would be to use the third option. And also this was my expectation of this function all along. I believe many people could expect such behaviour as well.

But in any case, this would be a breaking release. However, this is for the good 🙂