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/libstd/sys/unix/os.rs

@@ -505,3 +505,7 @@ pub fn home_dir() -> Option<PathBuf> {
         }
     }
 }
+
+pub fn exit(code: i32) -> ! {
+    unsafe { libc::exit(code as c_int) }
+}

src/libstd/sys/windows/c.rs

@@ -433,6 +433,7 @@ extern "system" {
                             TokenHandle: *mut libc::HANDLE) -> libc::BOOL;
     pub fn GetCurrentProcess() -> libc::HANDLE;
     pub fn GetStdHandle(which: libc::DWORD) -> libc::HANDLE;
+    pub fn ExitProcess(uExitCode: libc::UINT) -> !;
 }
 
 #[link(name = "userenv")]

src/libstd/sys/windows/os.rs

@@ -379,3 +379,7 @@ pub fn home_dir() -> Option<PathBuf> {
         }, super::os2path).ok()
     })
 }
+
+pub fn exit(code: i32) -> ! {
+    unsafe { libc::ExitProcess(code as libc::UINT) }
+}