Upgrade to syn 2

[leptonyu]

Do you have plan to upgrade dependencies?
A lots of dependencies are outdated.

[LucioFranco]

I have no come up with a plan yet but will need to figure this out for tonic as well and in addition will need to figure out what else needs to go in those releases. I don't have a timeline for this right now though.

[Xunjin]

I have no come up with a plan yet but will need to figure this out for tonic as well and in addition will need to figure out what else needs to go in those releases. I don't have a timeline for this right now though.

Is that hard to work on? Maybe create an issue so people could help. :P

[w-flo]

prost-build was upgraded to syn 2 in #833 over three months ago (not released yet), and there is an open PR for prost-derive #868 for some time.

So the plan that needs to be come up with is probably more about release planning, not the actual work to upgrade the syn version.