fix(ngOptions): ensure that the correct option is selected when options are loaded async

**Major reworking of select and ngOptions**:

* The `SelectController` is now used as an abstraction for the `select` and `ngOptions` directives
to override to get their desired behaviour
* The `select` directive is completely oblivious to the ngOptions directive now - the `ngOptions`
directive could be deleted without having to make any changes to the `select` directive.
* Select related directives (single/multiple/ngOptions) can provide specific versions of
`SelectController.writeValue` and `SelectController.readValue`, which are responsible for getting
the `$viewValue` in or out of the actual `<select>` element and its `<option>` children.

BREAKING CHANGE:

When using `ngOptions`: the directive applies a surrogate key as the value of the `<option>` element.
This commit changes the actual string used as the surrogate key. We now store a string that is computed
by calling `hashKey` on the item in the options collection; previously it was the index or key of the
item in the collection.

Before you might have seen:

```
<select ng-model="x" ng-option="i in items">
  <option value="1">a</option>
  <option value="2">b</option>
  <option value="3">c</option>
  <option value="4">d</option>
</select>
```

Now it will be something like:

```
<select ng-model="x" ng-option="i in items">
  <option value="string:a">a</option>
  <option value="string:b">b</option>
  <option value="string:c">c</option>
  <option value="string:d">d</option>
</select>
```

If your application code relied on this value, which it shouldn't, then you will need to modify your
application to accommodate this. You may find that you can use the `track by` feaure of `ngOptions`
as this provides the ability to specify the key that is stored.

Closes angular#9714

Author: petebacondarwin