Add chain, else* and cond*

[pyr]

Three functions to help build more concise streams.

• chain is a dummy function which helps you jump to a different
• cond* acts as a cond-like equivalent in streams
• else* is nice as the default operation for cond*

[pyr]

@aphyr,

Thanks for the review.

First about cond_, like your cond and condp, but I still think there should be a generic cond_ when you
want arbitrary fns, I meant to build cond on top of cond*, i can do that in a later PR if you want.

As for pipe, I like the pipeline idea, a lot. This is what i wanted to express with chain and how I am using it.
What I do that seems harder to express with a pipeline is skipping some steps which jumping allows me to do.

I'm not sure pubsub is as natural to read so I would be wary of using that.

I'm a bit worried about macro hell to be honest, and I think finding a way to decouple the predicate and filter funcs from call-rescue would be nice before moving on with this work. maybe by having streams wrap each of its forms, what do you think ?