you should be able to just remove the ref in Token = ref object. That is one layer of indirection less, and therefore faster and less memory consumption. And a little comment on what you are doing. Tokens are not nested. What you are doing is already parsing. In brainfuck all tokens are characters, so tokenization is basically irrelevant.
And the last part is setting current and latest. That is a very dirty way of programming. As soon as programs become more complex you will no longer know what side effects your functions have. It is better if you find a way to encode current and latest as arguments and return values. You don't need any mutable state, if you return at the top level a nested Token instead of seq. You should know that none of these recommendations are special Nim style, they are good programming style in general.
