make nextPermutation faster

[toyboot4e]

This PR makes nextPermutation like 10 times faster, resolving the TODO from 2014:

-                  -- TODO: make tuple unboxed
-                  let (kval',k') = if prev < cur then (prev,i-1) else (kval,k)
+                  let (!kval',!k') = if prev < cur then (prev,i-1) else (kval,k)

Let's do it!

Edit: stToPrim (suggested by @gksato) makes it even faster. With stToPrim, the bangs are not needed for the speed, but I preferred explicit strict evaluation.

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If anyone is interested, here's my AtCoder submissions for a simple speed comparsion:

• GM.nextPermutation: time limit exceed
• GM.nextPermutation with tuples unboxed: accepted (281 ms in the worst case)
• GM.nextPermutation with stToPrim and with tuples unboxed: accepted (231 ms in the worst case)

[Shimuuar]

LGTM. Extra strictness won't cause any problems since kval' and k' will be passed to strict function loop immediately

I make simple benchmark and this change reduced execution time 5× and allocations 9×. Most gains seems to come from stToPrim. Probably with simpler program GHC is better able pick up strictness.

@toyboot4e do you plan any more optimizations?

[gksato]

In our private conversation on Twitter, oh, X (in Japanese), I have suggested some optimizations to him/her, and (s)he suggested that I take this over. I'm submitting superceding PR.

[toyboot4e]

Thank you! Closing in favor of #498.