Tag trait objects with the 'dyn' keyword

The dyn keyword is now mandatory for trait objects (RFC 2113)
and rustc emits a warning where it is missing.

Author: phi-gamma

src/convert.rs

@@ -301,7 +301,7 @@ impl<'a, T: ToSongPath> ToSongPath for &'a T {
     }
 }
 
-impl ToSongPath for AsRef<str> {
+impl ToSongPath for dyn AsRef<str> {
     fn to_path(&self) -> &str {
         self.as_ref()
     }

src/error.rs

@@ -182,7 +182,7 @@ pub enum Error {
 pub type Result<T> = result::Result<T, Error>;
 
 impl StdError for Error {
-    fn cause(&self) -> Option<&StdError> {
+    fn cause(&self) -> Option<&dyn StdError> {
         match *self {
             Error::Io(ref err) => Some(err),
             Error::Parse(ref err) => Some(err),