Add __debugInfo() to managers

[Firehed]

A couple of the properties can contain enormous amounts of data that's unhelpful for debugging. Adding this magic method can trim that down pretty significantly.

Locally I found the following decent on balance:

InMemoryEntityManager:

    public function __debugInfo(): array
    {
        return [
            'repos' => $this->repos,
            'needIds' => $this->needIds,
            'pendingDeletes' => $this->pendingDeletes,
            'onFlushCallbacks' => sprintf('(%d functions)', count($this->onFlushCallbacks)),
        ];
    }

InMemoryRepository:

    public function __debugInfo(): array
    {
        return [
            'className' => $this->className,
            'idField' => $this->idField,
            'idType' => $this->idType,
            'isIdGenerated' => $this->isIdGenerated,
            'managedEntities' => $this->managedEntities,
        ];
    }