Contrib: waterfall(..)

[getify]

Need a waterfall(..) contrib plugin, which takes a list of functions, and applies them in order just like a normal sequence of then(..)s, but it collects aggregated output, where step 2 gets output from step 1, step 3 gets output from both step 2 and 3, step 4 gets messages from 1-3, etc.

The final success message(s) from waterfall(..) is the full aggregate of all success messages. An error anywhere along the way behaves like an error in a normal sequence.

Note: this method is sorta conceptually similar to an array-map, where we're mapping a set of steps to a set of messages.