Data.IntMap.Strict.findWithDefault is too strict

[Christian Maeder]

Hi,

when trying out Data.IntMap.Strict instead of Data.IntMap, I've noticed 
(after some confusion) that Data.IntMap.Strict.findWithDefault evaluates 
the default value, even if it is not used!

I usually pass an error-call to IntMap.findWithDefault to get a more 
informative crash-message than by using IntMap.!

Evaluating (error "...") surely crashes, but the evaluation should only
happen when the key is not in the map!

I can easily work around this (by using fromMaybe and IntMap.lookup), 
but it is still worth at least being documented if crucial for performance.

(the same applies to Data.Map)

Cheers Christian