Split AST::set_source into set_source/clear_source.

src/bin/rhai-repl.rs

@@ -92,7 +92,7 @@ fn main() {
                 .compile(&contents)
                 .map_err(|err| err.into())
                 .and_then(|mut ast| {
-                    ast.set_source(Some(&filename));
+                    ast.set_source(&filename);
                     Module::eval_ast_as_new(Default::default(), &ast, &engine)
                 }) {
                 Err(err) => {