Upgrade to Syn 2.0

[jsdw]

Now it's a thing. Mainly to long-term avoid having two copies of Syn in our dependency graph, but perhaps also it makes one or two things a little nicer after 3.5 years!

[ascjones]

Had a quick go at this since I was currently doing it in other places. However it is blocked on darling being updated so need to wait for that.