`useStore` doesn't re-render on `setKey` for `deepMap`

`useStore` isn't re-rendering when watching a specific key. If I have a setup like this: ```js const $store = deepMap({ foo: { counter: 1 }, bar: { counter: 1 } }); const Component = () => { const result = useStore($store, { keys: ['foo.counter'] }); const increment = () => { $store.setKey('foo.counter', $store.get().foo.counter + 1); }; } ``` then `result` is never updated. If I create a `listenKeys` listener I can see the store is changing correctly but `useStore` never triggers a re-render. [Codesandbox](https://codesandbox.io/s/recursing-mcclintock-rzfk57?file=/src/App.js) with the issue