Instead of returning internal generator results as ordinary JS::Objects
with properties, we now use GeneratorResult and CompletionCell which
both inherit from Cell directly and allow efficient access to state.
1.59x speedup on JetStream3/lazy-collections.js :^)
This avoids emitting TDZ checks for multiple bindings declared and
referenced within one variable declaration, i.e:
var foo = 0, bar = foo;
In the above case, we'd emit an unnecessary TDZ check for the second
reference to `foo`.
