Return error on composite key.

[kevincox]

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https://github.com/RReverser/serde-xml-rs/blob/d824bca2f193d554bfe58d5de211994c7079bf12/src/ser/var.rs#L49

IIUC this can be done by creating a new serializer that is restricted to only "key safe" types instread of returning parent. Then that serilizer throws an error for anything that can't be used as a key.