## Description

The `choices` field for the `ChoiceField` class should be able to be edited after `ChoiceField.__init__` is called.

```
field = ChoiceField(choices=[1,2])
field.choices = [1]  # Should no longer allow `2` as a choice
```

Currently, you must update `choices`, `grouped_choices`, and `choice_strings_to_values` to achieve this. This P/R keeps `grouped_choices` and `choice_strings_to_values` in sync whenever the `choices` are edited.