An simple enum variant extractor

Can extract a (mutable) reference or turn into a tuple from fields. When variant is different than the expected, return None.

Example

```rust use digenum::DigEnum; use std::io::Read;

[derive(DigEnum)]

enum Login { Local, Stream(T) Token([u8; 64]), UserAndPasswd(String, String), } ```

Maps to: | Variant | Reference | Mutable Reference | Owned | | ------- | --------- | ----------------- | ----- | | Local | None | None | None | | Stream(T) | as_Stream -> Option<&T> | as_mut_Stream -> Option<&mut T> | into_Stream -> Option<T> | | Token([u8; 64]) | as_Token -> Option<&[u8; 64]> | as_mut_Token -> Option<&mut [u8; 64]> | into_Token -> Option<[u8; 64]> | | UserAndPasswd(String, String) | as_UserAndPasswd -> Option<(&String, &String)> | as_mut_UserAndPasswd -> Option<(&mut String, &mut String)> | into_UserAndPasswd -> Option<(String, String)> |

"Why do not turn snake_case?"

Consider this example: ```rust use digenum::DigEnum;

[derive(DigEnum)]

enum Foo { FOO, foo, Foo } ``` How should I handle it?

Works on

• [x] Empty enums
• [x] Empty variants
• [x] Single-fielded variants
• [x] Multiple-fielded variants
• [x] Different-case variants (like Foo::FOO and Foo::foo)
• [x] Generic variants
• [x] Unit variants
• [ ] Struct-like variants