From India to Mexico and Brazil, the world’s biggest developing countries are witnessing some of the steepest economic contractions on record, throwing tens of millions out of work and turning back the clock on gains against poverty. Developing nations haven’t felt this kind of pain since the Great Depression. India’s economy shrank by nearly a quarter, 23.9%, during the April to June period compared with a year earlier, its worst performance since quarterly figures began in 1996. Peru’s economy contracted by 32% during that…