lookupDefault not strict in default value

I don't see any good reason to be strict in the default value of `lookupDefault`. It has nothing to do with the big-oh guarantees that motivate value strictness elsewhere, and defeats its principal use case, which is to *not* evaluate the potentially expensive fallback unless necessary. You shouldn't run through Plan B before you have even tried Plan A.

See discussion:
http://www.reddit.com/r/haskell/comments/1ker84/destroying_performance_with_strictness/