Left-associative fold of a structure but with strict application of
the operator.
This ensures that each step of the fold is forced to weak head normal
form before being applied, avoiding the collection of thunks that
would otherwise occur. This is often what you want to strictly reduce
a finite list to a single, monolithic result (e.g.
length).
For a general
Foldable structure this should be semantically
identical to,
foldl' f z = foldl' f z . toList