Convert syn to 0.15 and make it compile again

I updated the syn-version to 0.15.3. This was more work then anticipated, because there are a lot of breaking changes. It does come with a great reward: it gives better error handling for free.

Because of using the error messages in syn::parse::Error, I could remove the error-chain dependency (issue #30 (closed)). I made it simple for myself to just set the error on the nearest span I could lay my hands on.

I tried to make it compile on the beta-channel, but currently didn't succeed in that. I did manage to remove most of the feature-attributes.

Because I basically rewrote the parser, the edges of it can still be a bit rough. The unittest seems to succeed though.