[whatwg/streams] Factor out some logic into helper functions (#505)

As well as writable-stream.js, factor out promise manipulation and exception creation logic into helper functions.

This fixes wrong messages in ReadableStreamBYOBReader.

Align promise resolving helpers so that all of them don't take the fulfillment value but just use undefined.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/505

-- Commit Summary --

  * Factor out some logic into helper functions

-- File Changes --

    M reference-implementation/lib/readable-stream.js (194)
    M reference-implementation/lib/writable-stream.js (18)

-- Patch Links --

https://github.com/whatwg/streams/pull/505.patch
https://github.com/whatwg/streams/pull/505.diff

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