Deprecate one of token_list or opt_token_list

[madmongo1]

See #1886 for motivation.

Both types are essentially identical, with two irrelevant details. One type could be removed with no impact on users.

Note the member function token_list::existsand the free function validate_list.

[madmongo1]

I'm not entirely sure any more that opt_token_list and token_list share the same functionality

[vinniefalco]

Yep lets put a pin in this and revisit

[klemens-morgenstern]

@vinniefalco When's the revisting going to happen?

[vinniefalco]

We can look at it together when you are ready