This is in preparation for an upcoming implementation of the Iterator
Helpers proposal. That proposal will require a JS::Object type named
"Iterator", so this rename is to avoid conflicts.
Instead of using plain objects as Iterator records, causes confusion
about the object itself actually being its [[Iterator]] slot, and
requires non-standard type conversion shenanigans fpr the [[NextValue]]
and [[Done]] internal slots, implement a proper Iterator record struct
and use it throughout.
Also annotate the remaining Iterator AOs with spec comments while we're
here.
