Update to syn `0.12`

[KodrAus]

So we can report errors much more better using spans to the actual code that prevents auto_impl from being valid.

[LukasKalbertodt]

Hi @KodrAus,

I just wanted to let you know that I'm working on a syn update by rewriting the crate, basically. First I just tried to update it to the new version incrementally, but I noticed that quite a lot has happened since this crate was written. Thus, updating wasn't that easy and would have required a lot of cleanup. So I figured I could just try to rewrite it.

I'm already like... half done, approximately. Should be done in the next few days.

Would you (in principle) accept a PR like that?

[KodrAus]

Hi @LukasKalbertodt! Wow that sounds great to me! I'd be more than happy to just give you the commit bit so you can push changes through if you'd like.

I'm really interested to see what an implementation using syn 0.12 looks like.

[LukasKalbertodt]

@KodrAus I'd prefer to create a PR and having someone to at least quickly skim over the new code. But thanks for invitation anyway. I'm sure there are more updates needed before the proc_macro system hits stable :P

And if this wasn't clear: I upgraded to the newest version of syn (0.14.4), quote (0.6.3) and proc_macro2 (0.4.6). But yes, I think the new implementation will look a lot different than the old one.

I'll open a PR once I'm finished!