Deprecate Ember.merge in favor of Ember.assign

Summary

The goal of this RFC is to remove Ember.merge in favor of using Ember.assign.

Motivation

Ember.assign has been around quite awhile, and has the same functionality as Ember.merge. With that in mind, we should remove the old Ember.merge, in favor of just having a single function.

Detailed design

Ember will start logging deprecation messages that tell you to use Ember.assign instead of Ember.merge.

The exact deprecation message will be decided later, but something along the lines of:

Using `Ember.merge` is deprecated. Please use `Ember.assign` instead. If you are using a version of
Ember <= 2.4 you can use [ember-assign-polyfill](https://github.com/shipshapecode/ember-assign-polyfill) to make `Ember.assign`
available to you.

How we teach this

This should be a simple 1 to 1 conversion, and the deprecation message should be clear enough for all to understand what they need to do, and convert all usages of Ember.merge to Ember.assign.

Deprecation Guide

An entry to the Deprecation Guides will be added outlining the conversion from Ember.merge to Ember.assign.

Ember.merge predates Ember.assign, but since Ember.assign has been released, Ember.merge has been mostly unnecessary. To cut down on duplication, we are now recommending using Ember.assign instead of Ember.merge. If you are using a version of Ember <= 2.4 you can use ember-assign-polyfill to make Ember.assign available to you.

Before:

import { merge } from '@ember/polyfills';

var a = { first: 'Yehuda' };
var b = { last: 'Katz' };
merge(a, b); // a == { first: 'Yehuda', last: 'Katz' }, b == { last: 'Katz' }

After:

import { assign } from '@ember/polyfills';

var a = { first: 'Yehuda' };
var b = { last: 'Katz' };
assign(a, b); // a == { first: 'Yehuda', last: 'Katz' }, b == { last: 'Katz' }

Codemod

A codemod will be provided to allow automatic conversion of Ember.merge to Ember.assign.

Drawbacks

The only drawback, that I can think of, is people would need to convert Ember.merge to Ember.assign, but this would be a very easy change and could easily be done via codemod.

Alternatives

The impact of not doing this, is we continue to have two functions that do basically the same thing, which we need to maintain.

Another alternative, could be to remove both Ember.merge and Ember.assign, in favor of Object.assign or something similar.

Unresolved questions

None, that I can think of.