std: Add a process::exit function

This commit is an implementation of [RFC #1011][rfc] which adds an `exit`
function to the standard library for immediately terminating the current process
with a specified exit code.

[rfc]: https://github.com/rust-lang/rfcs/pull/1011

src/test/run-pass/process-exit.rs

@@ -0,0 +1,31 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::env;
+use std::process::{self, Command, Stdio};
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    if args.len() > 1 && args[1] == "child" {
+        child();
+    } else {
+        parent();
+    }
+}
+
+fn parent() {
+    let args: Vec<String> = env::args().collect();
+    let status = Command::new(&args[0]).arg("child").status().unwrap();
+    assert_eq!(status.code(), Some(2));
+}
+
+fn child() -> i32 {
+    process::exit(2);
+}