Create NativeCallable trait.

src/lib.rs

@@ -75,6 +75,7 @@ mod engine;
 mod error;
 mod fn_call;
 mod fn_func;
+mod fn_native;
 mod fn_register;
 mod module;
 mod optimize;
@@ -90,6 +91,7 @@ pub use any::Dynamic;
 pub use engine::Engine;
 pub use error::{ParseError, ParseErrorType};
 pub use fn_call::FuncArgs;
+pub use fn_native::NativeCallable;
 pub use fn_register::{RegisterDynamicFn, RegisterFn, RegisterResultFn};
 pub use parser::{AST, INT};
 pub use result::EvalAltResult;