Add parse_program to parser

The `Parser` can now parse a sequence of top-level expressions,
representing a complete program. This change modifies the `roundtrip`
function to iterate over these expressions and emit them, allowing for
multi-line program parsing.

src/main.rs

@@ -186,7 +186,7 @@ impl CompilerApp {
 
     fn roundtrip(&mut self) {
         let mut parser = Parser::new(&self.source_code);
-        let ast = parser.parse_expression();
+        let expressions = parser.parse_program();
         if parser.diagnostics.has_errors() {
             let errors: Vec<_> = parser
                 .diagnostics
@@ -198,7 +198,7 @@ impl CompilerApp {
             self.active_tab = AppTab::Output;
             return;
         }
-        self.source_code = emitter::emit(&ast);
+        self.source_code = expressions.iter().map(emitter::emit).collect::<Vec<_>>().join("\n");
         self.output_log = "Roundtrip complete — source reformatted.".to_string();
         self.active_tab = AppTab::Output;
     }