core: Move Exclusive and SharedMutableState to the private mod

src/test/run-pass/alt-ref-binding-in-guard-3256.rs

@@ -1,5 +1,5 @@
 fn main() {
-    let x = Some(unsafe::exclusive(true));
+    let x = Some(private::exclusive(true));
     match move x {
         Some(ref z) if z.with(|b| *b) => {
             do z.with |b| { assert *b; }