[streams] Don't allow negative-sized chunks (#370)

Closes #368. See previously #187 and f1a7e78d50274b34b556dff0d80b96e2a9207c60. We can always relax this later if someone has a use case.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/370

-- Commit Summary --

  * Don't allow negative-sized chunks

-- File Changes --

    M index.bs (2)
    M reference-implementation/lib/queue-with-sizes.js (4)
    M reference-implementation/test/bad-strategies.js (121)

-- Patch Links --

https://github.com/whatwg/streams/pull/370.patch
https://github.com/whatwg/streams/pull/370.diff

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https://github.com/whatwg/streams/pull/370