Refactor: Mv OsStr/OsStrExt to core, OsString/OsStringExt to alloc

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

Author: NobodyXu

Diff for: library/alloc/src/ffi/mod.rs

@@ -89,3 +89,18 @@ pub use self::c_str::{FromVecWithNulError, IntoStringError, NulError};
 
 #[unstable(feature = "c_str_module", issue = "112134")]
 pub mod c_str;
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[doc(inline)]
+pub use self::os_str::OsString;
+
+#[unstable(feature = "os_str_display", issue = "120048")]
+pub mod os_str;
+
+#[unstable(
+    feature = "os_str_internals",
+    reason = "internal details of the implementation of os str",
+    issue = "none"
+)]
+#[doc(hidden)]
+pub mod wtf8;