spawn a separate thread for builtin/custom pipe-out threads

there will be 3 child handles:
1. normal process child handle for external commands
2. sync function handle for builtin/custom commands without piping
3. thread spawning handle for builtin/custom pipe-out commands

fix #22

src/lib.rs

@@ -314,6 +314,7 @@ pub use logger::init_builtin_logger;
 pub use process::{export_cmd, set_debug, set_pipefail, Cmd, CmdEnv, Cmds, GroupCmds, Redirect};
 
 mod builtins;
+mod child;
 mod io;
 mod logger;
 mod process;