Missing field *length* in flow type in *fields*

Due missing field `length` in [fields](https://github.com/final-form/react-final-form-arrays/blob/master/src/types.js.flow#L25) by accessing it i get flow type error despite the fact that this [field](https://github.com/final-form/react-final-form-arrays/blob/master/src/useFieldArray.js#L94) exists.

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

Bug

### What is the current behavior?

Missing flow type for a field `length`

in here:
```
export type FieldArrayRenderProps = {
fields: {
...
length: number, // <<== this line need to be added
},
meta: Meta
}
```

### What is the expected behavior?

Need to add missing type

### What's your environment?
Latest version of Final Form Arrays

Open src/types.js.flow around line 25 and compare the fields shape with src/useFieldArray.js around line 94, where length is accessed. Update the Flow type so the existing length field is represented, then confirm the reported Flow type error is resolved.

Written by the indexing model from the issue text.