Proposal: Access to `app` instance synchronously

## Currently

Components can only access the `app` instance via `observe` HoC, which also involves RxJS for streaming props:

```js
import React from 'react';
import { observe } from 'frint-react';

function MyComponent() {
return

;
}

export default observe(function (app, parentProps$) {
return props$;
})(MyComponent);
```

## Proposal

Not everyone needs to work with streaming props, and may only want to be able to access the `app` instance (which has the providers), and carry on with regular React lifecycle operations.

To achieve that, we can extend `frint-react`:

### 1: `withApp` function

```js
import React from 'react';
import { withApp } from 'frint-react';

function MyComponent() {
return

;
}

export withApp((app, props) => );
```

### 2: `WithApp` component with render prop

This may not be necessary. Just `withApp` function alone should be enough.

```js
import React from 'react';
import { WithApp } from 'frint-react';

export default function MyComponent() {
return (

{
(app, props) =>

}

);
}
```