feat(core): field meta API

[harry-whorlow]

Field.meta

From issue #1111 and discussion #709, this pr Introduces the Field.meta api to allow for extensible and easily accessible meta data to the field component.

example api

<form.Field
  name="foo"
  meta={({ values }) => ({
    disabled: values.someOtherField == null,
    ...
  })}
>
  {(field, meta) => 
  <Comp
    disabled={meta.disabled}
    ... >
  </Comp>

---

Tasks

• [x] Meta available to Field.children
• [ ] MetaFn available to form.Field
• [ ] Tests
• [ ] LGTM 🚀