Add for statement and range function; New register_box_fn for functions returning Box<dyn Any> (which should not be boxed again); Register standard utility functions in library (e.g. print, string concat).

src/lib.rs

@@ -47,4 +47,4 @@ mod parser;
 
 pub use any::Any;
 pub use engine::{Engine, EvalAltResult, Scope};
-pub use fn_register::RegisterFn;
+pub use fn_register::{RegisterBoxFn, RegisterFn};