Revert "Simplify str::to_managed"

This reverts commit ac50046a.

src/libcore/str.rs

@@ -2262,8 +2262,9 @@ impl &str: StrSlice {
 
     #[inline]
     pure fn to_managed() -> @str {
-        let bytes = as_bytes_slice(self);
-        let v = at_vec::from_fn(bytes.len(), |i| bytes[i]);
+        let v = at_vec::from_fn(self.len() + 1, |i| {
+            if i == self.len() { 0 } else { self[i] }
+        });
         unsafe { ::cast::transmute(v) }
     }