[draft] Submit API

[secundant]

Proposal:

const formA = createForm({
  fields: {},
  // ...
  submit: () => fetch()
});

const formB = createForm({
  submit: {
    mapParams: fields => ({ foo: 'bar', ...fields }),
    fn: fetchFx
  }
});

const formC = createForm({
  submit: {
    source: { foo: $foo },
    mapParams: (fields, fromSource) => ({ ...fromSource, ...fields }),
    fn: fetchFx
  }
});

formC.$submitStatus; // Store<'initial' | 'pending' | 'done' | 'fail'>
formC.$succeeded; // Store<boolean>

formC.submitted.success; // Event<?>
formC.submitted.failure; // Event<?>
formC.submitted.finally; // Event<?>