Replace uses of 'unchecked' with 'unsafe'

src/test/compile-fail/borrowck-unchecked-with-borrow.rs

@@ -5,7 +5,7 @@ fn foo(v: &const Option<int>) {
     match *v {
       Some(ref i) => {
         //~^ ERROR illegal borrow unless pure
-        unchecked {
+        unsafe {
             impure(*i); //~ NOTE impure due to access to impure function
         }
       }