Combine operator(s)

[vitaly-t]

To combine the iterable with a list of iterables as parameters, into an array of values.

Two popular strategies for that:

• Stop the iterable when one of the iterables reaches the end - zip logic.
• Carry on for as long as at least one iterable hasn't finished - combine logic

[vitaly-t]

Operator zip has been added in v1.3.0

Now just need to add the combine operator.

[vitaly-t]

Started on combine within combine branch, but then parked it, because the logic for specifically asynchronous combine implementation isn't clear. In fact, I'm not sure that it is even possible, for iterables.

[vitaly-t]

Synchronous-only version of combine was added in v1.3.1, but no asynchronous. See #61

[vitaly-t]

This won't happen, so, closing.