Add POSIX::Spawn method for extra point of reference

Here is new output:

```
# Running benchmarks:


Process information retrieval            1              10             100            1000           10000
+bench_POSIX_Spawn_popen4         0.005980        0.025996        0.004227        0.002901        0.002780
bench_Process_stats               0.000048        0.000041        0.000034        0.000035        0.000036
bench_Process_stats_children      0.000042        0.000038        0.000034        0.000031        0.000032
bench_Process_stats_self          0.000048        0.000046        0.000032        0.000033        0.000035
bench_Spawn_ps_rss                0.050123        0.020579        0.043924        0.058219        0.064938
bench_System_uname                0.000042        0.000031        0.000027        0.000027        0.000026


Finished benchmarks in 0.673457s, 8.9093 tests/s, 8.9093 assertions/s.

6 tests, 6 assertions, 0 failures, 0 errors, 0 skips
```