bitfields-derive: Upgrade to syn 2

[langston-barrett]

syn is a big package that takes a long time to compile, and it recently had a major version bump. c2rust-bitfields-derive is a transitive dependency of my application that relies on syn 1, meaning I have to compile syn twice to compile my application. It would be great if bitfields-derive supported syn 2, so I only had to build syn once.

[domenukk]

+1

[bungcip]

current version of c2rust-bitfields-derived already upgraded to syn 2. I think we can close this issue.

[kkysen]

• Fixed by Upgrade to Syn 2 #1271.