Refactor derive_type

[leighmcculloch]

Similar to #266 but for derive_type that is used for contracttype macro.

[leighmcculloch]

Similar to #266 (comment) I think we should move the parsing logic into a type that implements syn::Parse.

I think it would be beneficial to then split the rendering logic into separate derive macros, where each derive macro reuses the same parsing logic. It would be ideal if we can only run the parsing logic once though, especially so we don't generate duplicate errors, so it might be that having them be entirely independent derive macros would be challenging, but otherwise they could be.

I'd think we should do this after and not before #153, and all the other tasks we have planned for macros currently.