Barbecue was born out of necessity, but now it’s a rich culture of pitmasters honing in on technical perfection and experimentation. Across the country, new restaurants are popping up that infuse worldwide cuisines with American traditions, while others prove that there’s magic in simplicity. In each case, these new spots are drawing local fans, hungry travelers, and Michelin Guide inspectors. First, a primer: Generally speaking, in this country, low and slow is the name of the game.