Allow for additional variants in destination

[RXminuS]

Woudl be great if there was a way of annotating that a case in the destination variant doesn't exist in the source variant. Right now I'd get a no variant or associated item named "XXX" found for enum since it tries to convert to an equally named variant from the source if I leave it un-annotated/empty.

[abrgr]

What do you think about adding an annotation to unused cases to preserve the default behavior of mapping by name? Maybe we use the existing from_case syntax but setting the source case to _ has the effect of shipping it? For example:

#[derive(FromEnum, Debug, PartialEq, Eq)]
#[from_enum(Src)]
enum Dest {
    Case1(),

    Case2(),

    #[from_case(_)]
    DestCase3 { a: DestStrField, b: u8 },
}