Performance via referential equality on useForm functions

### Are you submitting a **bug report** or a **feature request**?

**feature request** (or more like a **performance request**)

### What is the current behavior?

`onBlur`, `onChange` and `onFocus` from `useForm(...).input` do not maintain referential equality between renders. Therefore React.memo is hard to correctly implement.

### What is the expected behavior?

Preferably the aforementioned functions (`onBlur`, `onChange` and `onFocus`) would be referentially equal between renders.

### What's your environment?

```
"react-dom": "16.8.2"
"final-form": "^4.11.1"
"react-final-form-hooks": "^1.0.0"
```

### Other information