Could input and output streams be Duplexes?

[MichalCz]

Feature description

Currently the signature of a program is:

<T,U>(inp: ReadableStream<T>, ...args: any[]) => AnyReadable<U>

This means that all the programs we can support are unidirectional. That's cool, but makes a "hairpin" solution very tricky. We could make the developers' lives much easier by simply letting systems respond directly.

Is it loooads of work? @alicja-gruzdz @patuwwy