This patch rethinks the way indexed property storage works:
Instead of having a cut-off point at 200 elements where we always move
to generic property storage, we now allow arrays to stay in simple mode
as long as we don't create a gap/hole larger than 200 elements.
We also simplify generic storage to only have a hash map (for now)
instead of juggling both a vector and a hash map. This is mostly fine
since the vast majority of arrays get to stay simple now.
This is a huge speedup on anything that uses basic JS arrays with more
than 200 elements in them. :^)
