(rand-weighted ..args)
  args: two or more arguments (any type)
  returns any type

Randomly selects one item from a group, with unequal probability.

Expects an even number of arguments, where the even-numbered arguments are all non-negative numbers (weights). The sum of all weights must be greater than 0.

Each argument's likelihood of being chosen is equal to (/ weight sum). For example, (rand-weighted 'x 2 'y 3 'z 0) has a two-in-five chance of returning 'x and a three-in-five chance of returning 'y. It will never return 'z.