Consider updating encase_derive_impl to use syn@2

[LikeLakers2]

Hi! I noticed that encase_derive_impl (and possibly other parts of this repository) depend on syn@1. Perhaps you could consider updating to use syn@2 wherever possible?

Some things worth noting for anyone who intends to take on this suggestion:

• syn@2's MSRV is Rust 1.56, up from syn@1's Rust 1.31. I am unsure if this may be a problem - the repository's main Cargo.toml specifies a MSRV of Rust 1.63, but encase_derive and encase_derive_impl do not specify a MSRV.
• Updating to syn@2 may not be as simple as swapping the version number - there are a number of breaking changes in syn@2, listed at https://github.com/dtolnay/syn/releases/tag/2.0.0.

If you do consider to transition to syn@2, you have my thanks! :)

[teoxoy]

See #35

[LikeLakers2]

Ah, I must've missed that while checking through the repository. My bad!

[teoxoy]

No worries! Regarding the MSRV, it's the same for the derive crates as it is for the main lib.