Restore compatibility with rustc pre 1.53

The signature of NonNull::as_ref is more restrictive on lifetimes in
older standard libraries.

    error[E0515]: cannot return value referencing local variable `repr`
      --> src/rust_str.rs:19:9
       |
    19 |         repr.as_ref()
       |         ----^^^^^^^^^
       |         |
       |         returns a value referencing data owned by the current function
       |         `repr` is borrowed here
diff --git a/src/rust_str.rs b/src/rust_str.rs
index 02bd442..2f9c95b 100644
--- a/src/rust_str.rs
+++ b/src/rust_str.rs
@@ -16,7 +16,7 @@
 
     pub unsafe fn as_str<'a>(self) -> &'a str {
         let repr = mem::transmute::<RustStr, NonNull<str>>(self);
-        repr.as_ref()
+        &*repr.as_ptr()
     }
 }