Skynet 1M Fiber microbenchmark in D

[Per Nordlöw]

I'm curious about Fiber/coroutine performance in D compared to 
other languages such as Rust.

How should the following test be implemented in D?

Creates an actor (goroutine, whatever), which spawns 10 new 
actors, each of them spawns 10 more actors, etc. until one 
million actors are created on the final level. Then, each of them 
returns back its ordinal number (from 0 to 999999), which are 
summed on the previous level and sent back upstream, until 
reaching the root actor. (The answer should be 499999500000).

See also: https://github.com/atemerev/skynet