Case key paths

[stephencelis]

This branch reimagines case paths from first principles with the help of macros.

Given the new @CasePathable macro, one can annotate an enum:

@CasePathable
enum AppAction {
  case home(HomeAction)
  case settings(SettingsAction)
}

And get access to a bona fide key path for each case of the enum:

\AppAction.Cases.home  // CaseKeyPath<AppAction, HomeAction>

These case key paths can embed and extract values:

let appHomeCasePath = \AppAction.Cases.home

// Embed using function call syntax:
let action = appHomeCasePath(.onAppear)  // AppAction.home(.onAppear)

// Extract using a case subscript:
action[case: appHomeCasePath]  // Optional(HomeAction.onAppear)

// Extraction can fail:
action[case: \.settings]  // nil

And benefit from all the things key paths benefit from, like type inference, autocomplete, equatability, hashability, dynamic member lookup, and more.

Our libraries using case paths will be updated to support this new format soon. Be on the lookout!