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/dynamic_lib.rs

@@ -229,7 +229,7 @@ pub mod dl {
     }
 
     pub fn check_for_errors_in<T>(f: || -> T) -> Result<T, String> {
-        use rt::mutex::{StaticNativeMutex, NATIVE_MUTEX_INIT};
+        use rustrt::mutex::{StaticNativeMutex, NATIVE_MUTEX_INIT};
         static LOCK: StaticNativeMutex = NATIVE_MUTEX_INIT;
         unsafe {
             // dlerror isn't thread safe, so we need to lock around this entire