fix: support all accumulators in enum forward deserialization

[gibbz00]

Previously limited to Option only.

The following does not compile in main:

#[derive(Debug, PartialEq, ToXml, FromXml)]
#[xml(forward)]
enum FooCow<'a> {
    Bar(Cow<'a, [BarBorrowed<'a>]>),
    Baz(Cow<'a, [BazBorrowed<'a>]>),
}

[djc]

Nice work figuring this out, this crate isn't exactly well-documented...

Can you explain at a conceptual level why you want/need this?

[djc]

Fixing the pre-existing clippy issue in #72.

[gibbz00]

Nice work figuring this out, this crate isn't exactly well-documented...

Thanks! Yeah, at this stage it will have to be a dedicated effort for it to happen. Not a big deal right now since those looking at this crate in the first place will probably have a fairly good understanding of serde, which helps.

Can you explain at a conceptual level why you want/need this?

Sure! My task at work is to add support for secDNS per RFC 5910. (djc/instant-epp#17). The <info> part returns the same DsOrKeyData enum currently used by <create>. It could be reused with a slight semver bump if:

#[derive(Debug, ToXml)]
#[xml(forward)]
pub enum DsOrKeyData<'a> {
    DsData(&'a [DsDataType<'a>]),
    KeyData(&'a [KeyDataType<'a>]),
}

// is turned into this:

#[derive(Debug, ToXml, FromXml)]
#[xml(forward)]
pub enum DsOrKeyData<'a> {
    DsData(Cow<'a, [DsDataType<'a>]>),
    KeyData(Cow<'a, [KeyDataType<'a>]>),
}