move all OS-related code into src/os

src/lib.rs

@@ -27,5 +27,3 @@ mod arch;
 
 #[cfg(feature = "os")]
 pub mod os;
-#[cfg(feature = "os")]
-mod sys;

src/os/mod.rs

@@ -4,7 +4,7 @@ extern crate std;
 use core::prelude::*;
 use self::std::io::Error as IoError;
 use stack;
-use sys;
+mod sys;
 
 pub struct StackSource;