Add `of_seq` for lazy generation of data for pipeline

[rand00]

There already is an of_list function - I think it would be nice to have an of_seq too. This could be used to lazily generate data as the first node in a pipeline - e.g.

• synthetic data (i.e. no in-memory data to begin with),
• iterating through a datastructure,
• reading from a socket/pipe

[charlesetc]

Sure this sounds great to have! Added help wanted if you or anyone else wants to give it a try.