Allow compose transformer return arbitrary type

[wclr]

Currently compose requires that returned value was a stream of transformed values.

What about allowing to return arbitrary type of value?

const partition = <T>(predicate: (item: T) => boolean) =>
  (stream: Stream<T>) => [
    stream.filter(predicate),
    stream.filter(item => !predicate(item))
  ]

// this would allow such syntax
const [good$, bad$] = source$.compose(partition(filterGood))

// vs 

const [good$, bad$] = partition(filterGood)(source$)

Or you see compose as strictly Stream to Stream transformation function? (though JS without TS version still allows to do this)

[staltz]

@whitecolor This is interesting. I thought it would hurt typings elsewhere, but I gave it a try, and we can easily do this with zero difference for JS users, and no breaking changes for anyone (JS or TS users). It's just a backwards compatible feature. It's an easy win, so why not.

[staltz]

It's so easy, I just did it. Released in v10.9

[wclr]

Great. This allows more flexible operators. BTW what do you is partition operator needed in extras? I quite often find it in a need.