FieldMutability is missing Parse and ToTokens

[programmerjake]

I'm trying to write future-proof code that asserts matches!(field.mutability, FieldMutability::None) and reports a nice error if it isn't None:

let Field { mutability, .. } = &mut field;
if !matches!(mutability, FieldMutability::None) {
    errors.push(syn::Error::new_spanned(mutability, "field mutability is not supported"));
    *mutability = FieldMutability::None;
}

but that doesn't work because FieldMutability doesn't implement ToTokens.
Please add a ToTokens implementation. Adding a Parse implementation while you're at it would be nice.

[Fancyflame]

Have you considered using Span::call_site() as a temporary substitute, or the span of the whole field?