Replace many usages of useObservableEagerState with useBehavior

This hook is simpler in its implementation (therefore hopefully more correct & performant) and enforces a type-level distinction between raw Observables and Behaviors.

src/state/ObservableScope.ts

@@ -33,3 +33,8 @@ export class ObservableScope {
     this.ended$.complete();
   }
 }
+
+/**
+ * The global scope, a scope which never ends.
+ */
+export const globalScope = new ObservableScope();