Make lines stricter to fix a space leak

Proposal: A stricter implementation of lines. Reason: The current implementation causes a space leak (cf. http://homepages.inf.ed.ac.uk/wadler/papers/leak/leak.ps), at least in GHC. The proposed implementation fixes the leak at the small cost of being stricter if the first _|_ in the String is the first character of a line. Discussion period: Three weeks, until 15th October (because of ICFP). Ticket: http://hackage.haskell.org/trac/ghc/ticket/4334 Cheers, Daniel