Introduces CodeStubAssembler helpers for common Iterator operations
(GetIterator, IteratorStep, IteratorClose).

Moves the Promise.all resolveElement closure and it's caller to
builtins-promise-gen.cc.

Instead of creating an internal array (and copying its elements into a result
array), a single JSArray is allocated, and appended with BuildAppendJSArray(),
falling back to %CreateDataProperty(), and elements are updated in the resolve
closure the same way. This should always be unobservable.

This CL increases the size of snapshot_blob.bin on an x64.debug build by 11.44kb

BUG=v8:5343
R=cbruni@chromium.org, gsathysa@chromium.org, jgruber@chromium.org

Change-Id: Id69b7f76866b29caccd97f35870154c4be85f418
Reviewed-on: https://chromium-review.googlesource.com/497974
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45306}