Add comparison benchmark for out-parameter.
By value is now almost competitive:
```
Benchmark Mode Cnt Score Error Units
PointsAlloc.circle_by_ptr avgt 5 9.151 ± 0.196 ns/op
PointsAlloc.circle_by_ptr:·gc.alloc.rate avgt 5 233.423 ± 5.019 MB/sec
PointsAlloc.circle_by_ptr:·gc.alloc.rate.norm avgt 5 2.240 ± 0.001 B/op
PointsAlloc.circle_by_ptr:·gc.count avgt 5 6.000 counts
PointsAlloc.circle_by_ptr:·gc.time avgt 5 10.000 ms
PointsAlloc.circle_by_value avgt 5 15.445 ± 0.433 ns/op
PointsAlloc.circle_by_value:·gc.alloc.rate avgt 5 138.301 ± 3.855 MB/sec
PointsAlloc.circle_by_value:·gc.alloc.rate.norm avgt 5 2.240 ± 0.001 B/op
PointsAlloc.circle_by_value:·gc.count avgt 5 3.000 counts
PointsAlloc.circle_by_value:·gc.time avgt 5 6.000 ms
```
