Make most of std::rt private

Previously, the entire runtime API surface was publicly exposed, but
that is neither necessary nor desirable. This commit hides most of the
module, using librustrt directly as needed. The arrangement will need to
be revisited when rustrt is pulled into std.

[breaking-change]

src/libstd/io/stdio.rs

@@ -40,9 +40,9 @@ use option::{Option, Some, None};
 use boxed::Box;
 use sys::{fs, tty};
 use result::{Ok, Err};
-use rt;
-use rt::local::Local;
-use rt::task::Task;
+use rustrt;
+use rustrt::local::Local;
+use rustrt::task::Task;
 use slice::SlicePrelude;
 use str::StrPrelude;
 use uint;
@@ -207,7 +207,7 @@ fn with_task_stdout(f: |&mut Writer| -> IoResult<()>) {
         local_stdout.replace(Some(my_stdout));
         result
     } else {
-        let mut io = rt::Stdout;
+        let mut io = rustrt::Stdout;
         f(&mut io as &mut Writer)
     };
     match result {