Faster IntMap.size and IntSet.size

[meooow25]

I've been playing around with a few alternative implementations for IntMap.size to see if it can be made faster, and interestingly it seems it can.

Here are the implementations: https://gist.github.com/meooow25/0864ec79ad7885b470bf3d927b4deffa

And results on my machine with GHC 9.10.1:

  asc
    size:       OK
      2.08 μs ± 179 ns
    sizeNoNil:  OK
      1.66 μs ± 105 ns, 0.80x
    sizeUnroll: OK
      1.54 μs ± 124 ns, 0.74x
    size2:      OK
      1.33 μs ±  22 ns, 0.64x
    size3:      OK
      1.14 μs ± 102 ns, 0.55x
    size4:      OK
      1.16 μs ±  51 ns, 0.56x
    sizeStack2: OK
      1.49 μs ±  61 ns, 0.71x
  rand
    size:       OK
      2.17 μs ± 149 ns
    sizeNoNil:  OK
      2.02 μs ± 179 ns, 0.93x
    sizeUnroll: OK
      2.53 μs ± 188 ns, 1.17x
    size2:      OK
      1.52 μs ± 115 ns, 0.70x
    size3:      OK
      1.36 μs ± 108 ns, 0.63x
    size4:      OK
      1.56 μs ± 119 ns, 0.72x
    sizeStack2: OK
      1.43 μs ±  94 ns, 0.66x

Of course, these are still O(n) (#763), but being faster doesn't hurt.

size3 seems to be doing the best so we could switch to it, unless someone offers a faster implementation.