Add fine-grained reactivity for cursors

[red-meadow]

First of all, thank you very much for both Vue 2 and Vue 3 integrations!

The current approach with Tracker.autorun(() => { ... cursor.fetch() ... }) causes unnecessary invalidation of all objects in the array, even if only few of them were changed. Using cursor.observe() instead allows for precise invalidation. It also prevents unnecessary calls of transform function if it is passed to a cursor.

This approach is successfully used by Svelte and Blaze integrations.