Feature: Maps of

Simple usage:

```php
$address= new XmlString('<map><a>A</a><b>B</b></map>');
$map= $address->next(new Enclosing('/'))->next(new MapOf()));  // ['a' => 'A', 'b' => 'B']
```

Nesting:

```php
$address= new XmlString('<map><a>A</a><b>B</b><c><key>value</key></c></map>');
$map= $address->next(new Enclosing('/'))->next(new MapOf(['c' => new MapOf())));  // ['a' => 'A', 'b' => 'B', 'c' => ['key' => 'value']]
```