Extract ArrayAccessRead from ArrayAccess

Hello, internals!

Recently I was implementing some collections and realized that I cannot use
ArrayAccess for immutable collections without throwing
BadMethodCallException in offsetSet and offsetUnset. I also cannot
have an interface ReadonlyCollection extends ArrayAccess, because
offsetSet and offsetUnset are mutators and have TValue in a
contravariant position.

Here's the idea: let's extract ArrayAccess::offsetExists and
ArrayAccess::offsetGet methods into a separate ArrayAccessRead
interface (name can be different) and allow read operations $exists = isset($object['key']) and $value = $object['key'] for objects of classes
that implement this interface. ArrayAccess will then extend
ArrayAccessRead and add the remaining 2 methods. Looks like this change
should not break backward compatibility.

interface ArrayAccessRead
{
    public function offsetExists(mixed $offset): bool;
    public function offsetGet(mixed $offset): mixed;
}

interface ArrayAccess extends ArrayAccessRead
{
    public function offsetSet(mixed $offset, mixed $value): void;
    public function offsetUnset(mixed $offset): void;
}

// then interfaces of the doctrine/collections package might look like this:

namespace Doctrine\Common\Collections;

interface ReadableCollection extends Countable, IteratorAggregate,
ArrayAccessRead {}
interface Collection extends ReadableCollection, ArrayAccess {}

--
Regards, Valentin Udaltsov