Allow chain-able/async emitChange or bulk change

[MaurizioVacca]

At the current state, if you use emitChange to update the form state in quick succession, the updates enter a race condition where the last called wins. For example, given the following scenario:

emitChange('name', value);
emitChange('surname', value);

the second emitChange will override the first one, so name will be accidentally restored to the original value instead of being updated.

Possible solutions may be:

• allow to consume emit* as a chain-able/async resource, e.g. await emitChange(fieldName, value);
• add a "bulk update" emitter that can take a part of the form as a resource, e.g. emitBulkUpdate({key_1: value, key_2: value});