move OS specifics into sys

src/lib.rs

@@ -26,3 +26,5 @@ mod arch;
 
 #[cfg(feature = "os")]
 pub mod os;
+#[cfg(feature = "os")]
+mod sys;

src/os.rs

@@ -2,9 +2,7 @@ extern crate std;
 use core::prelude::*;
 use self::std::io::Error as IoError;
 use stack;
-
-#[cfg(unix)]
-#[path = "os/unix.rs"] mod sys;
+use sys;
 
 pub struct StackSource;
 

src/sys/mod.rs

@@ -0,0 +1,5 @@
+pub use self::imp::*;
+
+#[cfg(unix)]
+#[path = "unix.rs"]
+mod imp;