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.
It was previously holding a StringView, which was either a view into a
LibUnicode-generated string or a string passed from the user.
Intl.NumberFormat will need this record to hold internally-created
strings, so a StringView will not suffice (the way the steps are laid
out, that view will ultimately end up dangling).
This shouldn't be too wasteful since the StringView it was holding was
converted to a String eventually anyways.
To be consistent with the style in Temporal, let's move all AOs in Intl
to their object file, rather than splitting the AOs between prototype
and constructor files.
