[Zip] Reimplement to remove Sendable constraint and reduce Task creation

[twittemb]

Hi,

This issue is just to keep track that I'm in the process of refactoring the zip operator to follow the work bootstrapped by @FranzBusch.

The goals are:

• to remove the Sendable constraint on the base iterators
• to optimise the number of created tasks

I will implement zip state machines which roles will be to compute the next state (and the output) given a current state and an event (as in a Mealy state machine), and use those state machines in the zip async sequences.

I'll keep you posted.

[twittemb]

As the PR has been merged we can close this issue.
Thanks @phausler and @FranzBusch