[WIP] SwiftUIScreen with action sink

[square-tomb]

This PR demonstrates rewriting some Workflow-backed screens from UIKit to SwiftUI, significantly simplifying their implementations:

• LoginScreen from the TicTacToe example
• TodoEditScreen from Tutorial 5

It does so by:

1. Following the approach described in the Binding SwiftUI Views to Workflow Renderings design doc, replacing the Renderings' Screen conformances with SwiftUIScreen conformances.
2. Altering the Renderings themselves to hold a single, Equatable action sink rather than multiple callback functions, allowing the Rendering itself to derive Equatable.

Commits

The PR is designed to be read commit-by-commit:

c7d51b6: Introduce WorkflowSwiftUI types

Pastes in some types from previous Workflow SwiftUI experiments, nearly unchanged, that are discussed in the Binding SwiftUI Views to Workflow Renderings design doc. Most importantly:

• SwiftUIScreen, the protocol that a Screen may conform to if it wishes to provide a SwiftUI View in lieu of a ViewControllerDescription
• ObservableValue, which exposes Renderings from the Workflow to the SwiftUI View

2fbfc5e: Make LoginScreen a SwiftUIScreen

Replaces LoginScreen's Screen conformance with a SwiftUIScreen conformance. This already simplifies the implementation significantly, but the next few commits simplify it further.

0340049: Replace callbacks with action sink

Replaces the callback functions in LoginScreen with a single ScreenActionSink, allowing LoginScreen to derive Equatable. The also simplifies LoginWorkflow and its tests a bit.

f039b47, 7a9164a: Syntactic sugar

Introduces extensions on ObservableValue that make data bindings in the View more concise.

53fc65b: Make TodoEditScreen a SwiftUIScreen

Rewrite another screen to SwiftUI, taking advantage again of all types and extensions described above.

Further reading

#228 adds additional syntactic sugar for exposing two-way bindings to properties on the State.