[streams] Use "perform" for abstract ops that cannot throw (#366)

Closes #342, by making it clear that the promise-returning methods do not throw since they always call out to "perform"s or similar.

Also found one case of unnecessary exception guarding, and a few unnecessary ReturnIfAbrupts.

Leaves out ReadableByteStream for now, pending #361.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/366

-- Commit Summary --

  * Use "perform" for abstract ops that cannot throw

-- File Changes --

    M index.bs (109)
    M reference-implementation/lib/writable-stream.js (8)

-- Patch Links --

https://github.com/whatwg/streams/pull/366.patch
https://github.com/whatwg/streams/pull/366.diff

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