Fix strictness of foldl' & foldr'

[konsumlamm]

Since they used foldl/foldr, they were not actually strict. I also tried the versions used in base (using foldl for foldr' and foldr for foldl'), but they were worse in my micro-benchmark (I ran print $ foldl' (+) 0 [1..10_000::Int]/print $ foldr' (+) 0 [1..10_000::Int] and checked the number of reductions and memory usage), although the alternative foldl' actually uses less memory.

[augustss]

Oops. Thanks!