Support unnamed structs

[evaporei]

We still need to design it better: https://twitter.com/GirlGameDev/status/1306622222908510208

Handle it here: https://github.com/otaviopace/edn-derive/blob/99f36fe74d2b9698f9642ca0cc3853d16f86983c/src/structs.rs#L9

[evaporei]

For now, the prefered way is from:

struct Abc(usize, String);
Abc(5, "Hello".to_string())

To something like:

:Abc {0 5 1 "Hello"}

But we need edn-rs to support this as valid EDN.

[naomijub]

https://github.com/otaviopace/edn-derive/pull/25