Use raw pointer casts for slice, str's .as_ptr()

We can now use raw pointer casts `*const [T] as *const T` and
`*const str as *const u8` instead of .repr() for getting the
pointer out of a slice.

src/libcore/str/mod.rs

@@ -1894,7 +1894,7 @@ impl StrExt for str {
 
     #[inline]
     fn as_ptr(&self) -> *const u8 {
-        self.repr().data
+        self as *const str as *const u8
     }
 
     #[inline]