std: Internalize almost all of std::rt

This commit does some refactoring to make almost all of the `std::rt` private.
Specifically, the following items are no longer part of its API:

* DEFAULT_ERROR_CODE
* backtrace
* unwind
* args
* at_exit
* cleanup
* heap (this is just alloc::heap)
* min_stack
* util

The module is now tagged as `#[doc(hidden)]` as the only purpose it's serve is
an entry point for the `panic!` macro via the `begin_unwind` and
`begin_unwind_fmt` reexports.

src/libstd/rt.rs

@@ -0,0 +1,65 @@
+// Copyright 2013 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.
+
+//! Runtime services
+//!
+//! The `rt` module provides a narrow set of runtime services,
+//! including the global heap (exported in `heap`) and unwinding and
+//! backtrace support. The APIs in this module are highly unstable,
+//! and should be considered as private implementation details for the
+//! time being.
+
+#![unstable(feature = "rt",
+            reason = "this public module should not exist and is highly likely \
+                      to disappear",
+            issue = "0")]
+#![doc(hidden)]
+
+use borrow::ToOwned;
+use mem;
+use sys;
+use sys_common::thread_info::{self, NewThread};
+use sys_common;
+use thread::{self, Thread};
+
+// Reexport some of our utilities which are expected by other crates.
+pub use sys_common::unwind::{begin_unwind, begin_unwind_fmt};
+
+#[cfg(not(test))]
+#[lang = "start"]
+fn lang_start(main: *const u8, argc: isize, argv: *const *const u8) -> isize {
+    sys::init();
+
+    let failed = unsafe {
+        let main_guard = sys::thread::guard::init();
+        sys::stack_overflow::init();
+
+        // Next, set up the current Thread with the guard information we just
+        // created. Note that this isn't necessary in general for new threads,
+        // but we just do this to name the main thread and to give it correct
+        // info about the stack bounds.
+        let thread: Thread = NewThread::new(Some("<main>".to_owned()));
+        thread_info::set(main_guard, thread);
+
+        // Store our args if necessary in a squirreled away location
+        sys_common::args::init(argc, argv);
+
+        // Let's run some code!
+        let res = thread::catch_panic(mem::transmute::<_, fn()>(main));
+        sys_common::cleanup();
+        res.is_err()
+    };
+
+    if failed {
+        101
+    } else {
+        0
+    }
+}