[WIP] Implement pipeline constructor from exec vector

[yurinnick]

[hniksic]

This looks very useful, thanks!

Could we change from_vec to from_iter and have it accept any type that implements IntoIterator<Item = Exec>? That way it would still accept a Vec<Exec>, but would also accept any other iterable that produces Execs.

[AlexanderThaller]

Hello I would need this feature as well. Would it be ok if i make another pullrequest with the changes you recommended?

[hniksic]

@AlexanderThaller I have nothing against it, go ahead. The PR was marked WIP, so I didn't pay it full attention, but the change is quite straightforward.