Resolves #4711
Resolves an issue with createRecord failing when being called for a non-existent model
Fixes an issue found with toString implementation in #5117

Example Fail case prior to this PR:

let person = run(() => store.push({
    data: {
      type: 'person',
      id: '1'
    }
}));

person.constructor.modelName; // will be unset

Previously, we had overly complicated the process of looking up a model with a mixture of public and private and public-looking but private methods. This complexity resulted in there being multiple code-paths by which a ModelClass could be looked up without a modelName or in which no factory for the ModelClass would be found but we would still attempt to create an instance from a missing factory.

Much of this complexity stemmed from the desire to check whether a ModelClass existed. I have strengthened our private _hasModelFor implementation, and trimmed us down to a single public modelFor hook and a single private _modelFactoryFor hook to eliminate confusion and prevent these sorts of errors from occurring.

Deprecates

• _modelForMixin
• _modelFor
• modelFactoryFor