Make lines stricter to fix a space leak

[Daniel Fischer]

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