build(deps): upgrade syn crate

[Conaclos]

Summary

Upgrade to Syn 2.x. This brings many breaking changes. This is a painful migration.

For now, I still encounter some issue and as usual debugging macros is so hard...

It is no longer possible to use a keyword as an identifier in attributes. Thus, I renamed type into use_type for the partial attribute.

I also tried to use the more idiomatic way of writting attribute parsing. I used teh provded methods for error reporting. However this seems still very cryptic and unhelpful when something wrong happen. I still don't understand how to improve error reporting for macros...

Test Plan

CI must pass.

[ematipico]

Unless there are significant build time gains, I think this isn't needed for the time being.