Implement Error::source on IntoStringError

IntoStringError only implemented Error::cause, which is
deprecated. This implemements Error::source instead.
Error::cause will still work as before, thanks to the default
implementation.

src/libstd/ffi/c_str.rs

@@ -919,7 +919,7 @@ impl Error for IntoStringError {
         "C string contained non-utf8 bytes"
     }
 
-    fn cause(&self) -> Option<&dyn Error> {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
         Some(&self.error)
     }
 }