Syn update and 2021

[la10736]

Fix #187

[LikeLakers2]

Hey, since you mention the MSRV in the changelog, may I suggest documenting that elsewhere?

If nothing else, I would suggest using Cargo.toml's rust-version field. This is a field added in Rust 1.56 (which is your MSRV, and additionally is the first version to support edition = "2021"), which Cargo will parse upon trying to build your crate - and will exit with an error should the current Rust version not be at least that version.