[#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;
 

src/lib.rs

@@ -174,7 +174,7 @@ pub fn clear_logger() -> Result<*const dyn Logger, u8>
 #[cfg(feature="use_std")]
 pub fn set_logger<F>(create_logger: F)
    -> Result<(), u8>
-   where F: FnOnce() -> Box<Logger>
+   where F: FnOnce() -> Box<dyn Logger>
 {
    // I hate using closures, but it is the only way I
    // can think of to handle this right now.
@@ -189,7 +189,7 @@ pub fn set_logger<F>(create_logger: F)
 /// This will return a pointer to the Logger that
 /// was previously set.
 #[cfg(feature="use_std")]
-pub fn clear_logger() -> Result<Box<Logger>, u8>
+pub fn clear_logger() -> Result<Box<dyn Logger>, u8>
 {
    match clear_logger_base()
    {