Form fields' custom options should be validated when constructing the field

[m4tx]

In case we have form fields that have custom options that might conflict with each other (for instance, "minimum length" and "maximum length", where min > max), we should be able to generate an error when constructing a field, rather than in clean_value, for instance.

Technically, this is now possible by overriding FormField::with_options and just panicking there. Right now, we implement this trait with a macro (impl_form_field), so there are multiple options to solve the problem:

• Change the macro so that some additional code can be added to the FormField::with_options implementation
• Change the FormField::CustomOptions type to enforce validation before constructing an instance of CustomOptions somehow (this approach is slightly more complicated, because this would require some changes in the derive macro implementations, but is arguably more "correct")
• Come up with something else?

Context: #286 (comment)

[ElijahAhianyo]

@m4tx I moved the validation to the Field creation by introducing a ValidateOptions trait, which should be called before the field is created here #304 . What are your initial thoughts about this approach?