Added '$struct' tag to identify structs in newtype enum variants.

[gimait]

Trying out the serialization options included in quick-xml I came across an issue with the serialization of newtype variants in enums, where structs would have duplicated nodes when serialized. This relates to open issues like #183 and #346.

I believe there might be a better way to do this, but a very basic concept I came up with is to simply mark which enum variants contain a struct, and then skip adding an extra node for such variant.

This makes it for me for now, and maybe some others could benefit from it until we find the right way to do it.
I would like to look into an automatic way of doing this if someone has any ideas of how I could detect automatically if the given newvariant is a struct.

Let me know what you think, any feedback is welcome :)

[gimait]

I'm doing some more work on this for a project I'm working on, and in the meanwhile, I'm seeing other features that I'm missing for serde. I'll close this PR for now and create a new more complete one later on, including a proposal to include those features.