Move *nix specific implementation to separate module

src/sys/mod.rs

@@ -0,0 +1,13 @@
+use cfg_if::cfg_if;
+
+cfg_if! {
+    if #[cfg(unix)] {
+        mod unix;
+        pub use self::unix::*;
+    } else if #[cfg(windows)] {
+        mod windows;
+        pub use self::windows::*;
+    } else {
+        compile_error!("wasi-common doesn't compile for this platform yet");
+    }
+}