Add Deref and DerefMut derive for enums

[kiendang]

Resolves #388

Synopsis

Add Deref and DerefMut derive for enums. Only works on a single field of each variant of the enum and as long as the field types are compatible.

#[derive(Deref)]
enum NumRefEnum3 {
    Variant1 {
        #[deref(forward)]
        num: Box<i32>,
        useless: bool,
    },
    Variant2 {
        num: i32,
        #[deref(ignore)]
        useless: bool,
    },
    Variant3(i32),
}

See added docs and tests for more usage examples.

Solution

Make sure each of the enum variants contains 1 and only 1 enabled field, then generate a match block to extract the field. Checking if the field types are compatible are unnecessary since this case will fail compilation anyway.

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[kiendang]

The failed lints are unrelated. Fixed in #486.

[tyranron]

@kiendang thanks!