Support Hash of Futures for Promises.zip

Reading the guide for the Promises work in 1.1.x I came across the fantastic Concurrent::Promises.zip method.

It's interface, per the guide, expects an array of Futures and will return a final value of said array's values. This is beneficial, but what would make my (and I suspect many other's) code cleaner is to optionally accept a Hash of promises instead of an Array.

e.g.

map_of_work = {
  multiply: Concurrent::Promises.future { 3*2 },
  divide: Concurrent::Promises.future { 3/2 },
  add: Concurrent::Promises.future { 3 + 2 },
  subtract: Concurrent::Promises.future { 3 - 2 },
}
# => {:multiply=>#<Concurrent::Promises::Future:0x00007fc474fad8a0 pending>,
#     :divide=>#<Concurrent::Promises::Future:0x00007fc474fad008 pending>,
#     :add=>#<Concurrent::Promises::Future:0x00007fc474fa7d60 pending>,
#     :subtract=>#<Concurrent::Promises::Future:0x00007fc474fa6ca8 pending>}
Concurrent::Promises.zip(map_of_work).value!
# => {:multiply=>6,
#     :divide=>1,
#     :add=>5,
#     :subtract=>1}

Why? Because it's a common pattern in Javascript libraries[1][2], and thus my team.

Arrays are slightly more cumbersome where I have to remember that index 2 is always for the addition work. Should another developer prepend more Futures to the array, we must not forget to change all the indexes referenced thereafter. With hashes, an :add is always an :add.

The entry point is Concurrent::Promises.zip; start by reading its current array-oriented behavior and the linked Promises guide. Support the shown hash-of-promises input while preserving keys and resolving values, then confirm the example returns the expected keyed result.