Detect arbitrary function calls with potential side effects in `no-side-effects` rule

Attempt to catch function calls in computed properties which are likely causing side effects.

```js
myProperty: computed(function() {
...
someFunctionCall(); // either useless or causes side effects
...
setLocalStorageItem('my-storage-key', 'true'); // causes side effects
...
return ...
});
```

Look for any statement within the computed property function that does not cause a mutation to a local binding. That’s easy for assignment but harder for e.g. `array.push`.

The existing rule [no-side-effects](https://github.com/ember-cli/eslint-plugin-ember/blob/master/docs/rules/no-side-effects.md) only catches `this.set('property', ...)` right now.

Related: #329