Redesign syntax as a proc macro

[brson]

There are a lot of deficiencies with the current syntax that are pretty hard to resolve as a macro_rules macro:

• Make it possible to attach attributes to Error and ErrorKind types
• Make it possible to attach visibility specifiers
• The syntax not matching corresponding declaration form
• The types declaration, with its hand-rolled system of defaults, doesn't quite feel "rustic"
• Attributes only coming after items
• No ability to place attributes or visibility modifiers on Error or ErrorKind
• No ability to use generics in error variants
• Cannot document fields in struct type error
• Cannot currently use derive on the Error type
• Links with custom description/display
• Make it possible to specify Send / Sync
  • I personally think it should be send / sync by default

This is a lot of constraints and demands a complete redesign.

[Yamakaky]

Maybe take inspiration from/merge with https://github.com/Arnavion/derive-error-chain

[Arnavion]

Previous discussion of merging derive-error-chain into this repository is at Arnavion/derive-error-chain#11

If the goal is to completely replace the existing macro_rules macro with a custom derive, then my concerns in that issue do not apply and I would be happy to merge its code here.