add parallel benchmark

When many UUIDs are being generated concurrently,
contention on the atomic counter can slow things down.

There might be ways to speed this up, but for now, just
add a parallel benchmark so we can measure the baseline.

Initial results with `go test -bench  . -cpu 1,2` on my machine (two physical cores):

	BenchmarkContended     	56413502	        18.3 ns/op
	BenchmarkContended-2   	82533951	        33.5 ns/op

Note that the 33.5ns/op measure is worse than it appears, because
that's wall ns/op, not cpu-ns/op (see golang/go#31884), so the time is actually 67.0ns/op when contended.

uuid_test.go

@@ -48,3 +48,12 @@ func BenchmarkNext(b *testing.B) {
 		g.Next()
 	}
 }
+
+func BenchmarkContended(b *testing.B) {
+	g := MustNewGenerator()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			g.Next()
+		}
+	})
+}