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/sys/unix/stack_overflow.rs

@@ -38,7 +38,7 @@ impl Drop for Handler {
           target_os = "openbsd"))]
 mod imp {
     use super::Handler;
-    use rt::util::report_overflow;
+    use sys_common::util::report_overflow;
     use mem;
     use ptr;
     use sys::c::{siginfo, sigaction, SIGBUS, SIG_DFL,