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/task.rs

@@ -50,9 +50,9 @@ use kinds::{Send, marker};
 use option::{None, Some, Option};
 use boxed::Box;
 use result::Result;
-use rt::local::Local;
-use rt::task;
-use rt::task::Task;
+use rustrt::local::Local;
+use rustrt::task;
+use rustrt::task::Task;
 use str::{Str, SendStr, IntoMaybeOwned};
 use string::{String, ToString};
 use sync::Future;
@@ -142,13 +142,13 @@ impl TaskBuilder {
             stack_size: stack_size,
         };
         if stdout.is_some() || stderr.is_some() {
-            spawner.spawn(opts, proc() {
+            Task::spawn(opts, proc() {
                 let _ = stdout.map(stdio::set_stdout);
                 let _ = stderr.map(stdio::set_stderr);
                 f();
             })
         } else {
-            spawner.spawn(opts, f)
+            Task::spawn(opts, f)
         }
     }
 
@@ -237,7 +237,7 @@ pub fn try_future<T:Send>(f: proc():Send -> T) -> Future<Result<T, Box<Any + Sen
 /// Read the name of the current task.
 #[stable]
 pub fn name() -> Option<String> {
-    use rt::task::Task;
+    use rustrt::task::Task;
 
     let task = Local::borrow(None::<Task>);
     match task.name {
@@ -249,7 +249,7 @@ pub fn name() -> Option<String> {
 /// Yield control to the task scheduler.
 #[unstable = "Name will change."]
 pub fn deschedule() {
-    use rt::task::Task;
+    use rustrt::task::Task;
     Task::yield_now();
 }
 
@@ -257,7 +257,7 @@ pub fn deschedule() {
 /// destructor that is run while unwinding the stack after a call to `panic!()`).
 #[unstable = "May move to a different module."]
 pub fn failing() -> bool {
-    use rt::task::Task;
+    use rustrt::task::Task;
     Local::borrow(None::<Task>).unwinder.unwinding()
 }