Allow callable as alternative to custom mapper

Nobody has claimed this yet.

It would be nice to be able to map an entire object with a callable/closure, as an alternative to creating a custom mapper. E.g. the following:

```php
$config->registerMapping(SomeClass::class, SomeOther::class)
->useCustomMapper(new class extends CustomMapper {
public function mapToObject($source, $destination)
{
// do stuff
return $destination;
}
});
```

Would become something like this:

```php
$config->registerMapping(SomeClass::class, SomeOther::class)
->fromCallable(function ($source, $destination) {
// do stuff
return $destination;
}});
```

Optionally the mapper can be passed as the third parameter. On a related note, the `CustomMapper` could maybe use a `MapperAwareInterface` as well.

Start by reading the registerMapping and useCustomMapper APIs, then inspect CustomMapper and the related MapperAwareInterface idea. Define how a callable alternative and optional third mapper parameter should work, and verify that a callable can map the source into the destination as shown.

Written by the indexing model from the issue text.