codegen: add Grouping expr support

- entry: return value of rayoko main

examples/hello.ry

@@ -26,7 +26,7 @@ fn multwo(num: i32, double_flag: bool) i32 {
 }
 
 fn add(a: i32, b: i32) i32 {
-    return a + b;
+    return a + b + (2 * b);
 }
 
 fn main() i32 {

src/codegen.zig

@@ -258,6 +258,10 @@ pub const Codegen = struct {
                 };
             },
 
+            .Grouping => |expr_ptr| blk: {
+                break :blk try self.emitExpr(builder, expr_ptr);
+            },
+
             else => {
                 std.debug.warn("Got unexpected expr {}\n", ast.ExprType(expr.*));
                 return CompileError.EmitError;

src/entry.c

@@ -1,3 +1,3 @@
 int main(void) {
-    __rayoko_main();
+    return __rayoko_main();
 }