Typings

There was a tonne of helpful feedback from @briancavalier on the exploratory API, trade-offs and internal workings I plan to look into. One thing he mentioned that would be a great first issue for someone else to pick is adding typings to the library.

If you apply an operator on an iterable source, the resulting composed emitter is also iterable. However, not every conceivable emitter is iterable and/or async iterable. This should be easy to statically identify and would make it easier for people to learn.

This sort of information can also be leveraged internally for optimisations, which I wasn't aware of e.g. simplifying a chain of emitters without any visible difference.