Use `TMB:::FreeADFun` to make sure external pointers are getting removed

[stevencarlislewalker]

Need to discourage user-managed ad functions.

• [ ] Don't export tmb_fun
• [ ] Remove user-exposed function arguments that accept ad functions
• [ ] Place ad functions in model objects
• [ ] Refresh ad functions inside the model when necessary
• [ ] Refresh functionality should include pointer removal from old function
• [ ] Pointer removal should be a global option with default to remove pointers

[stevencarlislewalker]

This has turned out to not be associated with any practical problems