more error handling

src/codegen_x86_64.rs

@@ -746,6 +746,7 @@ _builtin_environ:
             Expr::New(struct_name) => {
                 let struct_fields = &self.analyzer.structs[&struct_name.lexeme];
 
+                // TODO: panic on mem.alloc error
                 let memory_size = struct_fields.len() * 8;
                 emit!(&mut self.output, "    mov rdi, {}", memory_size);
                 emit!(&mut self.output, "    call mem.alloc");