feat(serde): Enable serialization & deserialization for Template*

[grenewode]

• Implement serde::Serialize and serde::Deserialize for Template, TemplateBuf, ByteTemplate and ByteTemplateBuf
• Add serde feature to crate, to control implementation of serde::Serialize and serde::Deserialize
• make json, toml and yaml features respect serde feature

[de-vri-es]

Thanks! I think this makes sense.

I think the implementation could be simplified a bit though. Rather than defining visitors you can just deserialize into a &'a str, String or byte equivalent and then parse them into a template.

Will save us a ton of code :)

[de-vri-es]

The Eq can also be derived.

[de-vri-es]

I've simplified the implementations a bit by getting rid of custom visitors where possible.

I kept one for ByteTemplateBuf because Vec<u8> (de)serializes inefficiently by default (&[u8] is already fine by itself).

I don't want to use serde_test though, as it's not maintained. And it doesn't test the parsed template in detail: an implementation that sets the source but doesn't actually parse it would be accepted to (since PartialEq only compares the source).

I don't know of a great alternative now though. For deserializing there's serde::de::value::{StringDeserializer,BorrowedStrDeserializer,BorrowedBytesDeserializer}, but noticably missing is something for OwnedBytesDeserializer.

Also, there is nothing equivalent for serializing.