Add function expr parsing

* Introduce new `fn` keyword
* Function example is added to examples/expr.not

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile/thunk.rs

@@ -345,6 +345,11 @@ impl Visit for CompileBody<'_> {
         Ok(thunk)
     }
 
+    fn visit_fun_expr(&mut self, _expr: &FunExpr) -> Self::Out {
+        // TODO : fun exprs should be compiled as constants and put into the constant pool
+        todo!()
+    }
+
     fn visit_atom(&mut self, atom: &Atom) -> Self::Out {
         let thunk = match atom {
             Atom::Ident(ident) => {