Fix derive(Decode) for enums with lifetime parameters and struct-like variants

[ImplOfAnImpl]

For an enum like this:

enum Enum<'a> {
    StructLike {
        val: u8
    },
    ... // Some other variants that use 'a
}

deriving Decoode will currently produce the compile-time error "lifetime arguments are not allowed on variant StructLike".

This is because the generated code creates an enum instance using the syntax Enum::<'a>::StructLike { ... } which the Rust compiler doesn't like (specifically, it doesn't like it when the generic parameter is a lifetime AND the variant is struct-like, but in other cases it doesn't mind).

Fortunately, the syntax EnumName::VariantName::<GenericParam> ... works in all cases.