isModified helper

Feature Proposal

It would be great to have isModified helper, much like isDraft or current functions.

Motivation

More efficient updates of derived data in draft. Immer currently do have all necessary information, in it's essence it is stripped down version of current.

Can this be solved in user-land code?

It can be solved in following ways:

1. You can compare current with original in draft - but that's unnecessarily expensive for simple yes/no query
2. You can finishDraft, compare with original, then createDraft again if necessary

Example

produce(deeplyNestedObject, (draft) => {
	performOpaqueDeeplyNestedUpdates(draft)
	
	if(isModified(draft.item)){
		draft.item.modifiedOn = new Date()
	}
})