Drop syn in graphql_client_codegen?

[tomhoule]

We don't use the nice syn features about reporting errors. The code we generate cannot be mapped back to a precise rust code span, since it depends on the GraphQL query. This would let us drop syn, quote and proc macro 2 dependencies from the CLI.

[tomhoule]

Yes, we should do this.

[tomhoule]

Let's drop syn but not quote.

[tomhoule]

After having a good look at it, it's not easy to replace, and it adds guarantees in the public api of graphql-client-codegen, so let's keep it in for now.