Try smaller segment length in binary fuse build

Some sizes around segment length transitions require many iterations
and would work much better with the previous segment length.

We add a simple fix that is more robust than tweaking the formula:
once every four iterations, we try the previous segment length while
keeping the same capacity. Note that in most cases this won't affect
the build because it's rare to need more than 1-2 iterations.

`TestBinaryFuseBoundarySizes` output (with numTrials=100):
```
binaryfusefilter_test.go:490: size: 2  iterations: 1.02 avg (2 max)
binaryfusefilter_test.go:490: size: 8  iterations: 1.08 avg (3 max)
binaryfusefilter_test.go:490: size: 24  iterations: 1.08 avg (3 max)
binaryfusefilter_test.go:490: size: 27  iterations: 1.03 avg (2 max)
binaryfusefilter_test.go:490: size: 55  iterations: 1.02 avg (2 max)
binaryfusefilter_test.go:490: size: 91  iterations: 1.02 avg (2 max)
binaryfusefilter_test.go:490: size: 120  iterations: 1.04 avg (2 max)
binaryfusefilter_test.go:490: size: 303  iterations: 1.04 avg (2 max)
binaryfusefilter_test.go:490: size: 349  iterations: 1.01 avg (2 max)
binaryfusefilter_test.go:490: size: 1009  iterations: 1.01 avg (2 max)
binaryfusefilter_test.go:490: size: 1124  iterations: 1.16 avg (4 max)
binaryfusefilter_test.go:490: size: 3361  iterations: 1.03 avg (2 max)
binaryfusefilter_test.go:490: size: 3551  iterations: 2.05 avg (6 max)
binaryfusefilter_test.go:490: size: 11192  iterations: 1.04 avg (3 max)
binaryfusefilter_test.go:490: size: 11521  iterations: 2.10 avg (6 max)
binaryfusefilter_test.go:490: size: 37272  iterations: 1.01 avg (2 max)
binaryfusefilter_test.go:490: size: 37454  iterations: 2.09 avg (6 max)
binaryfusefilter_test.go:490: size: 124117  iterations: 1.03 avg (2 max)
binaryfusefilter_test.go:490: size: 126131  iterations: 1.53 avg (4 max)
binaryfusefilter_test.go:490: size: 413309  iterations: 1.00 avg (1 max)
binaryfusefilter_test.go:490: size: 416077  iterations: 1.50 avg (4 max)
binaryfusefilter_test.go:490: size: 1376321  iterations: 1.02 avg (3 max)
```

Author: RaduBerinde

binaryfusefilter.go

@@ -90,6 +90,26 @@ func buildBinaryFuse[T Unsigned](b *BinaryFuseBuilder, keys []uint64) (_ BinaryF
 			// probability (i.e., a cosmic ray corrupts your system).
 			return BinaryFuse[T]{}, iterations, errors.New("too many iterations")
 		}
+		if size > 4 && size < 1_000_000 {
+			// The segment length is calculated using an empirical formula. For some
+			// sizes, the segment length is too large and leads to many iterations.
+			// Once every four iterations, use the previous segment length while
+			// keeping the same capacity. See TestBinaryFuseBoundarySizes.
+			switch iterations % 4 {
+			case 2:
+				// Switch to smaller segment size.
+				filter.SegmentLength /= 2
+				filter.SegmentLengthMask = filter.SegmentLength - 1
+				filter.SegmentCount = filter.SegmentCount*2 + 2
+				filter.SegmentCountLength = filter.SegmentCount * filter.SegmentLength
+			case 3:
+				// Restore the calculated segment size.
+				filter.SegmentLength *= 2
+				filter.SegmentLengthMask = filter.SegmentLength - 1
+				filter.SegmentCount = filter.SegmentCount/2 - 1
+				filter.SegmentCountLength = filter.SegmentCount * filter.SegmentLength
+			}
+		}
 
 		blockBits := 1
 		for (1 << blockBits) < filter.SegmentCount {