[Macros] Implement a type wrapper transformation as a macro.

[hborla]

The type wrapper macro struct conforms to MemberDeclarationMacro, MemberAttributeMacro, and AccessorDeclarationMacro. Each macro capability provides a different part of the type wrapper transform:

• MemberDeclarationMacro adds the backing var _storage variable.
• MemberAttributeMacro applies macro attributes to each stored property inside the wrapped type, which are recursively expanded.
• AccessorDeclarationMacro uses the macros applied to stored properties to add get and set accessors, turning those stored properties into computed properties that indirect access through _storage.

[xedin]

Just FYI that type wrapper is a bit more then just wrappedKeyPath:, it also detects whether it’s attached to a reference type and if so, adds wrappedSelf: argument and always storageKeyPath: as the last argument which points to \$Storage.<prop-name>.

[hborla]

I know, this is just a proof of concept. The macro can generate whatever it wants in the accessors. For example, if the macro is attached to an actor or class, it could apply a different macro attribute to each of the stored properties to direct the recursive expansion to generate special accessors in that case. It could also just choose to put whatever code is in the subscript implementation directly into the accessors

[xedin]

Sounds good!

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test

[hborla]

@swift-ci please test