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/lib.rs

@@ -162,7 +162,6 @@ pub use core::result;
 pub use core::option;
 
 pub use alloc::boxed;
-
 pub use alloc::rc;
 
 pub use core_collections::slice;
@@ -247,7 +246,7 @@ pub mod fmt;
 
 #[path = "sys/common/mod.rs"] mod sys_common;
 
-mod rt;
+pub mod rt;
 mod failure;
 
 // A curious inner-module that's not exported that contains the binding