Rust idiomatic binding to wait4

``` use std::process::Command; use wait4::Wait4;

let cmd = Command::new(command);

// ... let mut child = cmd.spawn().unwrap();

let r = child.wait4().unwrap(); ```

Status

• [x] Unix-like using libc::wait4
• [ ] Windows using winapi::um::processthreadsapi::GetProcessTimes and winapi::um::psapi::GetProcessMemoryInfo.