Conform BindableState to CustomDebugStringConvertible (#777)

This ensures that bindable state properties are displayed in a useful way when using
`.dump` snapshot tests on your app state.

Author: lukeredpath

Sources/ComposableArchitecture/SwiftUI/Binding.swift

@@ -228,6 +228,12 @@ import SwiftUI
 //    }
 //  }
 
+  extension BindableState: CustomDebugStringConvertible where Value: CustomDebugStringConvertible {
+    public var debugDescription: String {
+        self.wrappedValue.debugDescription
+    }
+  }
+
   /// An action type that exposes a `binding` case for the purpose of reducing.
   ///
   /// Used in conjunction with ``BindableState`` to safely eliminate the boilerplate typically