feat(serde)!: use internal serialization to roundtrip crate types

[martinohmann]

BREAKING CHANGE: hcl::ser::Serializer does not implement serde::Serializer anymore. Use serializer.serialize(&value) in places where value.serialize(&mut serializer) was used before.

This essentially uses the approach that the author of minijinja describes in this blog post: https://lucumr.pocoo.org/2021/11/14/abusing-serde/

I originally read this before I even added serialization support to hcl-rs and decided back then to go without internal serialization.

However, the number of types involved to represent the HCL structural, expression and template languages now makes it necessary to switch to internal serialization since each type needs a custom serializer implementation just for the sake of roundtripping it through serde. The amount of code that I wrote to make this work is already quite massive and would even get bigger once serialization of template via serde is fully supported. Also it was quite messy.

Thus, I switched to internal serialization to roundtrip all expression and structure types. This meant that I could remove a lot of custom serializers.

This also had some positive side effects:

• The #[serde(rename = "$hcl::...")] markers could be removed almost everywhere since they were only needed to make the roundtripping through serde work. The only type that still has this marker is Body since we need to special-case it during deserialization.
• The serializer code is much cleaner now.
• Serialization of types that are subject to roundtripping is quite a bit faster.
• It is now possible (and this change already implements most of it) to properly support serialization of HCL blocks (with or without labels) from custom types. The public API and documentation for this will be added as a followup. The custom_blocks test in src/structure/ser/tests.rs should give a first hint how this will work.