Improve foldl and foldr for IntSet

[treeowl]

foldrBits was previously defined just like foldr'Bits, and
foldlBits was defined just like foldl'Bits. That is, each of them
built up an accumulator, lazily, across a whole leaf before producing
anything. Swap that around so the folds can process values promptly.

Fixes #666

[treeowl]

I've done a bit of benchmarking. My results aren't very stable, but it seems, generally, that foldr is pretty similar (perhaps because of the cost of reversing the bits) while foldl is improved. Perhaps someone better at benchmarking and/or micro-optimization should take a look at it.

[treeowl]

Er.... no, that's not it. The reversing happens with foldl.... I have no explanation.

[treeowl]

@oisdk, this looks like the sort of thing you might know how to think about and test properly. Any ideas?

[meooow25]

@treeowl do you want to continue with this PR? The changes to foldrBits and foldlBits seem ok to me.
Once rebased we could add some benchmarks similar to what we recently added for Data.Tree's foldr and measure if there's an improvement.

[treeowl]

I'm currently recovering from COVID, so can't really do much this week.

[meooow25]

Hope you get well soon! I didn't mean to imply any sort of urgency, this can surely wait.

[treeowl]

I feel bad for making you wait so long on multiple things.

[meooow25]

I have had some free time, don't worry about it 😅