Enum Evolution

It seems highly desirable to allow adding and removing enum variants in a backwards compatible way.

There are two ways to support this:

1. Unknown variant: Enums may optionally have a unit variant marked `#[data(unknown)]`. When a decoder encounters an unknown variant, it returns that variant.

```rust
#[derive(Data)]
enum Foo {
Bar,
#[data(unknown)]
Unknown
}
```

2. Enum decoding always returns `Option`: Functions that decode enums always have the signature `fn load(...) -> Option`. If the decoder encounters an unknown variant, it returns `None`.

The downside of 1. is that you'll have to match against and handle the `Unknown` variant. The downside of 2. is that a double-option would be required to indicate a value not being present: `fn load(...) -> Option>`. The outer option is `None` if the enum isn't present, and the inner option is `None` if an unknown variant was encountered.

Review the existing enum decoding and load APIs, then compare the proposed #[data(unknown)] variant with Option-based decoding. Done means the project has selected and implemented a backwards-compatible behavior for unknown enum variants, including how absent values are represented.

Written by the indexing model from the issue text.