Simple wrapper for linux `wait4` syscall to get resource usage of a child process. This allows to measure multiple parameters of a child process, including CPU user/system times, memory usage, number of page faults, etc.
```rust
use command_rusage::GetRUsage;
use std::process::Command;
let mut child = Command::new("sleep").arg("1").spawn().unwrap();
let rusage = child.wait_for_rusage().unwrap();
println!("User time: {}s", rusage.utime.as_secs_f64());
```