BRUSSELS — Islamic extremists struck Tuesday in the heart of Europe, killing at least 34 people and wounding scores of others in back-to-back bombings of the Brussels airport and subway that again laid bare the continent’s vulnerability to suicide squads. Bloodied and dazed travelers staggered from the airport after two explosions — at least one blamed on a suicide attacker and another reportedly on a suitcase bomb — tore through crowds checking in for morning flights.