Fix Typo in CMakeLists.

Add basic nonsense grammar.

grammar/grammar.y

@@ -0,0 +1,19 @@
+%include {
+
+#include <iostream>
+#include <string>
+#include <assert.h>
+
+}
+
+%token_type {int}
+
+%left PLUS MINUS.
+
+%syntax_error {
+    std::cout << "Syntax error!" << std::endl;
+}
+
+program ::= expr.            { }
+
+expr ::= expr PLUS expr.     { }