Update syn, quote & proc-macro2 dependencies

[mwilliammyers]

Now that #16 is merged, we should probably update the syn, quote and proc-macro2 dependencies as they have all reached 1.0.0.

I will probably have time to tackle this...

[idanarye]

Reading the list of breaking changes at https://github.com/dtolnay/syn/releases/tag/1.0.0, I think that the only relevant change is the tts -> tokens rename.