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/thread/mod.rs

@@ -167,10 +167,11 @@ use any::Any;
 use cell::UnsafeCell;
 use fmt;
 use io;
-use rt::{self, unwind};
 use sync::{Mutex, Condvar, Arc};
 use sys::thread as imp;
 use sys_common::thread_info;
+use sys_common::unwind;
+use sys_common::util;
 use time::Duration;
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -260,7 +261,7 @@ impl Builder {
                                        -> io::Result<JoinInner<T>> {
         let Builder { name, stack_size } = self;
 
-        let stack_size = stack_size.unwrap_or(rt::min_stack());
+        let stack_size = stack_size.unwrap_or(util::min_stack());
 
         let my_thread = Thread::new(name);
         let their_thread = my_thread.clone();
@@ -383,7 +384,7 @@ pub fn catch_panic<F, R>(f: F) -> Result<R>
     let mut result = None;
     unsafe {
         let result = &mut result;
-        try!(::rt::unwind::try(move || *result = Some(f())))
+        try!(unwind::try(move || *result = Some(f())))
     }
     Ok(result.unwrap())
 }