func MakeWeightedItems(items []WeightedItem) WeightedItems {
	wi := WeightedItems{
		items: items,
	}
	for _, item := range items {
		wi.totalWeight += item.Weight
	}
	return wi
}
MakeWeightedItems creates a slice of WeightedItems and calculates
the total weight.
package random
type WeightedItem struct {
	Value  interface{}
	Weight int
}
type WeightedItems struct {
	items       []WeightedItem
	totalWeight int
}