Vision for macros in StateTree

[adam-zethraeus]

There's some clearly-related experimentation at https://github.com/adam-zethraeus/DictionaryBackedMacro

Any implementation would be api-breaking at a time when that's quite undesirable — so vision/plan required here first.

[adam-zethraeus]

Thoughts:

• use @Node annotation instead of conformance
• provide default empty var rules
• add transaction(_:) and run(behavior:) methods, deprecating @Scope
• make fields as values by default
  • require @Projection fields to be explicitly marked as 'ancestor-owned'
  • perhaps add 'unmanaged' annotation to capture the current stored-var behavior. iff its behavior can be well defined.