add length function to builtins.

README.md

@@ -57,8 +57,8 @@ Simply call the `./mumble` binary:
 ```
 mumble> (1 2 3)
 mumble> { 1 2 3 }
-mumble> head (1 2 3)
-mumble> { head (1 2 3) }
+mumble> head {1 2 3}
+mumble> { head {1 2 3) }
 mumble> tail { 1 2 3 }
 mumble> list ( 1 2 3 )
 mumble> eval { head {1 2 3} } 

src/mumble.c

@@ -1001,6 +1001,7 @@ void lispenv_add_builtins(lispenv* env)
     lispenv_add_builtin("tail", builtin_tail, env);
     lispenv_add_builtin("eval", builtin_eval, env);
     lispenv_add_builtin("join", builtin_join, env);
+    lispenv_add_builtin("len", builtin_len, env);
     lispenv_add_builtin("def", builtin_def, env);
     lispenv_add_builtin("@", builtin_define_lambda, env);
     lispenv_add_builtin("ifelse", builtin_ifelse, env);