[streams] Investigate merging RS and RBS (#379)

The work in #361 moves a lot of the interesting work into the controller class, making the outer RBS class fairly simple. RBS becomes a shell that delegates to RBSC.

One possibility this opens up is that we could remove RBS, making RS just a shell which delegates to _either_ one of RSC or RBSC. This could have good ecosystem benefits by not splitting the ecosystem into two different types of streams.

To do this I think we need to:

- [ ] Finish up #361.
- [ ] Port RS to a similar infrastructure.
- [ ] See if merging is possible and desirable.

Related: #312.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/379