Please update proc_macro2, quote, and syn to v1.0

[nbraud]

Dear serde maintainers,

While packaging the newest versions of proc-macro2, quote, and syn in Debian (which all recently reached 1.0), I discovered that serde doesn't yet support those version.

I am attempting to make a patch for this, but there's a lot to fix, so help would be appreciated.

[dtolnay]

For now you may be able to cherrypick #1591, if that's an option for you.

[nbraud]

Thanks @dtolnay, I had missed that PR (probably only searched through open issues)

I ended up picking all the commits (except the rust-format one) since v1.0.98, since it didn't seem worth trying to backport only the necessary change. Ran into some unrelated issues, but I can hopefully fix serde_derive in Debian tomorrow :)