[whatwg/streams] Change promises-guide references to Web IDL (#1021)

The spec text has broken links to the old [promises guide](https://www.w3.org/2001/tag/doc/promises-guide/). This is inconvenient for readers, but it also causes builds for spec PRs to fail (see [my comment on #1011](https://github.com/whatwg/streams/pull/1011#issuecomment-549029385)).

This PR fixes the links by pointing to their new Web IDL counterparts.

Todo list (non-exhaustive, feel free to suggest other changes):
* [ ] Get spec to build again.
* [ ] Figure out what to do with "promise-calling".
* [ ] Change usages of "react to promise" to explicitly pass one or two sets of steps (as in [this example](https://heycam.github.io/webidl/#promise-example-addDelay)), instead of passing a "fulfillment handler" and a "rejection handler".
* [ ] Change pairs of "upon fulfillment / upon rejection" to a single "react to promise".

Fixes #1020
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/1021

-- Commit Summary --

  * Change promises-guide references to webidl
  * Use "getting a promise to wait for all"

-- File Changes --

    M index.bs (24)

-- Patch Links --

https://github.com/whatwg/streams/pull/1021.patch
https://github.com/whatwg/streams/pull/1021.diff

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