Synchronisation on multiple futures missing

[supercooldave]

Oftentimes we see code like:

for f <- this.futures do
    get(f)
end

to synchronise a bunch of parallel computations.
This can be very expensive, especially if one or more of the futures have not been fulfilled.
A more asynchronous solution would involve building a chain of these futures and a single get. Perhaps there are better solutions. In any case, we need library support for this, appropriately benchmarked.

A version of the required operation is in the Task module. This uses await, so is unlikely to be efficient. (Of course, benchmarking would be required to answer that question.)