Upgrade syn crate to 2.0.45 version

[tyranron]

Related to #301

Synopsis

syn 2.0.45 fixes the #301 issue:

Otherwise this would not work:

#[derive(derive_more::Debug)]
enum Enum {
    A = if cfg!(unix) { 2 } else { 3 },
}

error: proc-macro derive panicked
 --> src/main.rs:1:10
  |
1 | #[derive(derive_more::Debug)]
  |          ^^^^^^^^^^^^^^^^^^
  |
  = help: message: called `Result::unwrap()` on an `Err` value: Error("unsupported expression; enable syn's features=[\"full\"]")

Solution

Upgrade minimal version of syn up to 2.0.45 version and cover the use case with tests for future regressions.

Checklist

• Documentation is updated (not required)
• Tests are added/updated
• CHANGELOG entry is added (not required)