The fact that a `MarkedValueList` had to be created was just annoying,
so here's an alternative.
This patchset also removes some (now) unneeded MarkedValueList.h includes.
Finally use Symbol.iterator protocol in language features :) currently
only used in for-of loops and spread expressions, but will have more
uses later (Maps, Sets, Array.from, etc).
Not only is this a much nicer api (can't pass a typo'd string into the
get_well_known_symbol function), it is also a bit more performant since
there are no hashmap lookups.
With the addition of symbol keys, work can now be done on starting to
implement the well-known symbol functionality. The most important of
these well-known symbols is by far Symbol.iterator.
This patch adds IteratorPrototype, as well as ArrayIterator and
ArrayIteratorPrototype. In the future, sometime after StringIterator has
also been added, this will allow us to use Symbol.iterator directly in
for..of loops, enabling the use of custom iterator objects. Also makes
adding iterator support to native objects much easier (as will have to
be done for Map and Set, when they get added).
