Proposal: make minimumBy/maximumBy go through foldl', not foldr1

[Roman Cheplyaka]

I just realized (thanks to this reddit thread[1]) that minimumBy is now
implemented through foldr1. Previously (before FTP), minimumBy from Data.List
used foldl1, while minimumBy from Data.Foldable used foldr1.

I think that the way these were merged was a mistake.

Let's admit that the absolute majority of use cases for minimumBy are numbers,
for which the comparison functions are strict in both arguments. Even for
something like Bool, where comparison could potentially be in one of the
arguments, the standard compare method is derived and is strict.

Now, for Foldable, there isn't necesarily a connection between the direction and
the way fold is performed. For example, for Data.Map.Map both folds are
tail-recursive. Yet foldr is non-tail-recursive at least for:

- good old lists
- anything that uses the same convention for foldl/foldr as lists
- anything that defines its foldable instance through conversion to lists
- anything that uses the default implementation of foldr/foldr1

I also put a simple microbenchmark[2] to show the difference in performance;
on it, the proposed implementation is more than 20x faster than the current one.

[1]: https://www.reddit.com/r/haskell/comments/3qpefo/a_very_unfortunate_error_message/
[2]: http://lpaste.net/714261956202070016

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