Use `Error::source` instead of `Error::cause` in `lightmotif-io`

lightmotif-io/src/error.rs

@@ -50,7 +50,7 @@ impl Display for Error {
 }
 
 impl std::error::Error for Error {
-    fn cause(&self) -> Option<&dyn std::error::Error> {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match self {
             Error::InvalidData => None,
             Error::Io(e) => Some(e),