Add a ChoicesMeta.max_length property returning the max_length when data type supports len()

[Coumes Quentin]

It would be helpful for enums inheriting Choices with a data type supporting len() to be able to compute max_length by themselves.

This would make the definition of field's max_length easier as well as automatically create a new migration when deleting/adding members which would change the max_length.

If the data type does not support len(), it could raise TypeError (same exception raise by len) something like :

raise TypeError("Can only compute max_length on data type supporting len()")

I can do the patch if this feature does make sense to you.