roaring: use DirectAdd rather than op.apply for cheap performance win

Calling op.apply on an op we know to be an add ends up noticably
increasing the cost of the operation; this trivial change gets about
a 5-10% reduction in reported runtime of benchmarks doing a lot
of adds. (The other IntersectionCount benchmarks don't actually use
Add most of the time, so it doesn't show up in them.)

name                                    old time/op  new time/op  delta
GetBenchData-8                          4.25ms ± 0%  3.91ms ± 2%   -8.04%  (p=0.002 n=6+6)
Bitmap_IntersectionCount_ArrayArray-8   20.9µs ± 2%  18.7µs ± 3%  -10.18%  (p=0.004 n=5+6)
SliceAscending-8                        24.7ms ± 0%  21.8ms ± 0%  -11.74%  (p=0.004 n=5+6)
SliceDescending-8                       29.8ms ± 0%  27.0ms ± 0%   -9.56%  (p=0.004 n=5+6)
SliceAscendingStriped-8                 32.0ms ± 0%  29.5ms ± 0%   -8.07%  (p=0.008 n=5+5)
SliceDescendingStriped-8                39.3ms ± 1%  36.8ms ± 1%   -6.27%  (p=0.002 n=6+6)

roaring/roaring.go

@@ -159,9 +159,8 @@ func (b *Bitmap) Add(a ...uint64) (changed bool, err error) {
 		}
 
 		// Apply to the in-memory bitmap.
-		if op.apply(b) {
+		if b.DirectAdd(v) {
 			changed = true
-
 		}
 	}