nat-n

# Construct a chained array of callbacks that "thread" a sequence of async functions in the smallest possible code.
# tldr; call the next async function in fn, "wait" for its callback to store the result and call the next async callback in cb
# each async function
#   accepts 1 callback argument
#   passes 1 arg of result data to the callback
# construct the callback
#   when callback in cb evaluates, replaces itself in cb array with the result
#   calls the next callback in cb
#   after last callback in cb, return joined results stored in cb