COR-1759 Serde deprecated

[AHaliq]

Purpose

Guard serde relevant constructs behind a feature flag eventually to be turned off by default

Changes

• created a tool to parse and transform rust AST to add the guards idempotently
• run cd serde-refactor && cargo run -- ../src --dry-run to see candidates in src
• apply transformation to src/** and examples/**

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

CLA acceptance

By submitting the contribution I accept the terms and conditions of the
Contributor License Agreement v1.0

• link: https://developers.concordium.com/CLAs/Contributor-License-Agreement-v1.0.pdf

• I accept the above linked CLA.

[allanbrondum]

Let's document also here in Cargo.toml that the flag means

[allanbrondum]

We can be a bit more specific about the flag. Instead of "use of serde", it is serde::Serialize/Deserialize implementation that are guarded by the flag. And only on types where we consider it deprecated and it might be removed in the future.

[allanbrondum]

I don't think we need to feature flag the serialize/deserialize functions here, since they do not really promise a "default" serialization format of a specific type. They only have effect when used explicitly. Hence, I think we can just not feature flag them.

[allanbrondum]

There are formatting changes and some removed comments that should be reverted in this file

[AHaliq]

was the previous formatter cargo +nightly-2023-04-01 fmt perhaps?