Feature: Bindings

This pull request implements bindings that can be passed to the `Injector`'s constructor:

```php
$inject= new Injector(
  new ApplicationDefaults(),
  new PersistenceContext(),
  newinstance('inject.Bindings', [], [
    'bind' => function($injector) { $injector->bind('string', 'World', 'Hello'); }
  ])
);
```