DAKAR, Senegal — A crowd of people holding curved metal spikes jumped on trash spilling out of a dump truck in Senegal’s biggest landfill, hacking at the garbage to find valuable plastic. Nearby, sleeves rolled up, suds up to their elbows, women washed plastic jerrycans in rainbow colors, cut into pieces. Around them, piles of broken toys, plastic mayonnaise jars and hundreds of discarded synthetic wigs stretched as far as the eye could see, all ready to be sold and recycled. Plastic waste is exploding in Senegal, as in many…