Added tsx generic typing for FieldArray

[ChadLefort]

Depends on:
final-form/react-final-form#522
final-form/final-form#225

This should allow for types to be passed to a FieldArray like so:

<FieldArray<{ firstName: string, lastName: string }> name="people">
  {({ fields: { value } }) => {
      // value will have a type now
  }}
</FieldArray>

[erikras]

Published in v3.1.0.