Avoid excessive view computation with @Bindable (#123)

Because our `@Bindable` holds onto a reference, this reference is probed
by SwiftUI to determine if a view body should be recomputed, and since
the creation of the view is going to create a whole new reference, it is
invalidating the view and causing it to re-render.

Instead, lets treat these references as equal if the objects they wrap
are equal. This doesn't make things completely 1:1 with SwiftUI, as
there is an initial invalidation in the parent, but a single extra
invalidation seems like a good improvement.

Fixes #122.

Sources/PerceptionCore/Bindable.swift

@@ -63,4 +63,10 @@
       self.object = object
     }
   }
+
+  extension Observer: Equatable where Object: AnyObject {
+    static func == (lhs: Observer, rhs: Observer) -> Bool {
+      lhs.object === rhs.object
+    }
+  }
 #endif