[heycam/webidl] Use an "end of iteration" signal instead of undefined (#885)

The use of undefined as a sentinel for the end of iteration was problematic for value iterators, where the possible value space could contain undefined.

@TimothyGu, could you take a look?

There's another issue lurking here which is that promises should only be resolved with either Web IDL values (if these are Web IDL `Promise<T>`s) or JS values (if these are JS promises). I considered adding a note explaining that end-of-iteration could be represented as a unique Web IDL/JS object with identity, and a pair could be represented as an appropriate Web IDL dictionary/JS null-prototype object, but I couldn't decide what type of promise these were. I guess we can figure that out as a followup, if we really want to.
You can view, comment on, or merge this pull request online at:

  https://github.com/heycam/webidl/pull/885

-- Commit Summary --

  * Use an "end of iteration" signal instead of undefined

-- File Changes --

    M index.bs (10)

-- Patch Links --

https://github.com/heycam/webidl/pull/885.patch
https://github.com/heycam/webidl/pull/885.diff

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