Fix bug where accessibility change resets context (#53)

Resolves #51

Creates and stores the context on the `AnyExhibit` model before view body calculation. This ensures that when swiftUI re-runs the body, we maintain the context for each exhibit.

![Kapture 2022-03-07 at 12 33 43](https://user-images.githubusercontent.com/609274/157113509-19ab2e95-4be8-415a-84ab-8304009915f6.gif)

Sources/Exhibition/Exhibit.swift

@@ -39,6 +39,7 @@ public struct AnyExhibit {
     let name: String
     let section: String
     let content: (Context) -> AnyView
+    let context = Context()
 
     init<Content: View, Layout: View>(_ exhibit: Exhibit<Content>, layout: @escaping (Content) -> Layout) {
         self.name = exhibit.name
@@ -59,6 +60,11 @@ struct AnyExhibitView: View {
     let exhibit: AnyExhibit
     @ObservedObject var context: Context
 
+    init(exhibit: AnyExhibit) {
+        self.exhibit = exhibit
+        self.context = exhibit.context
+    }
+
     var body: some View {
         exhibit.content(context)
     }

Sources/Exhibition/ExhibitListView.swift

@@ -79,8 +79,7 @@ public struct ExhibitListView: View {
     }
 
     private func debuggable(_ exhibit: AnyExhibit) -> some View {
-        let context = Context()
-        return AnyExhibitView(exhibit: exhibit, context: context)
+        return AnyExhibitView(exhibit: exhibit)
             .toolbar {
                 ToolbarItem {
                     Button {
@@ -92,7 +91,7 @@ public struct ExhibitListView: View {
             }
             .sheet(isPresented: $exhibitDebugViewPresented) {
                 DebugView(
-                    context: context,
+                    context: exhibit.context,
                     preferredColorScheme: $preferredColorScheme,
                     layoutDirection: $layoutDirection
                 )