core: Emit dead code warning via stderr

nac3core/src/typecheck/function_check.rs

@@ -312,7 +312,7 @@ impl<'a> Inferencer<'a> {
         let mut ret = false;
         for stmt in block {
             if ret {
-                println!("warning: dead code at {:?}\n", stmt.location)
+                eprintln!("warning: dead code at {:?}\n", stmt.location)
             }
             if self.check_stmt(stmt, defined_identifiers)? {
                 ret = true;