Safeguard

First attempt at a safeguard system.

1. These commits replace all calls to `eval` in the code with calls to `litable--safe-eval`.
2. This function takes a form as an argument.
3. It takes this form (which would usually be blindly evaluated) and
   checks each function being called inside it. If any of these
   functions is not listed in `litable-pure-functions-list`, the form
   is NOT evaluated and an informative message is shown.
4. While this message is being shown, the user can add these functions
   to this list by calling `litable-accept-as-pure`.