Have faker respect Rule.valueOfField(siblingField)

[saiichihashimoto]

After #336, valueOfField no longer breaks, but faker doesn't generate with them. Ideally, faker would include all validations. It's difficult to layer in the validations at the field level, because the containing object isn't included (and doesn't exist yet, considering the field is part of the definition). We'd have to generate the object schema, then go back through each field and find the validations that used valueOfField() to go and do those, which is somewhat of a back and forth. Might be worth it to ignore validations all together initially, generate the whole object, then layer the validations for each field on top of it. Would have to double check this works fine for objects of objects.