[#1] Fixed the use std feature build for Rust 2021.

This will bring the library into compliance with the new Rust
2021 version.

This was a simple change of how trait objects are done.

examples/logger.rs

@@ -47,7 +47,7 @@ impl TestLogger
    }
 
    #[cfg(feature="use_std")]
-   fn set_logger() -> Box<Logger>
+   fn set_logger() -> Box<dyn Logger>
    {
       let logger: TestLogger;