feat: support generics of PHP internal classes

[romm]

The mapper learns about generics by reading the @template annotations of a class docblock. Classes internal to PHP, or provided by an extension, carry no such docblock and cannot be modified, so the type of the values they hold could never be described.

Their signature is now known to the mapper, and the ones a source can be mapped to without ambiguity are built with no configuration:

final class Article
{
    public function __construct(
        public string $title,
        /** @var \Ds\Set<string> */
        public \Ds\Set $tags,
    ) {}
}

(new \CuyZ\Valinor\MapperBuilder())->mapper()->map(Article::class, [
    'title' => 'Some article',
    'tags' => ['php', 'valinor'],
]);

An array builds ArrayObject, ArrayIterator and Ds\Map, a list the Spl* and Ds\* collections, a key and a value Ds\Pair. The others are described but never built, until a constructor is registered for them: SplHeap is abstract, SplObjectStorage is keyed by objects, the priority queues need a priority for each value, Generator, WeakMap and WeakReference have no meaningful source shape, and the interfaces are given an implementation through MapperBuilder::infer().

Every template declares a default, so that these classes stay usable without generics, as they appear bare in countless existing signatures. Mapping to one without them now reports that mixed is not allowed in strict mode, where map(SplStack::class, []) previously returned an empty stack.

DateTime, DateTimeImmutable and DateTimeZone each had a builder factory of their own doing the same job; both are dropped, their constructors joining the list. A built-in constructor applies when no registered one takes a single argument, so customising the supported date formats no longer adds one back when a registered constructor already takes the source.

Fixes #529