This patch adds a `-z` option to js and test-js. When run in this mode,
garbage cells are never actually destroyed. We instead keep them around
in a special zombie state.
This allows us to validate that zombies don't get marked in future GC
scans (since there were not supposed to be any more references!) :^)
Cells get notified when they become a zombie (via did_become_zombie())
and this is used by WeakContainer cells to deregister themselves from
the heap.
Almost everything in LibJS includes Cell.h, don't force all code to
include AK/TypeCasts.h + AK/String.h. Instead include them where they
are actually used and required.
Previously, EnvironmentRecord was a JS::Object. This was done because
GlobalObject inherited from EnvironmentRecord. Now that this is no
longer the case, we can simplify things by making EnvironmentRecord
inherit from Cell directly.
This also removes the need for environment records to have a shape,
which was awkward. This will be removed in the following patch.
So far we only have two states: Live and Dead. In the future, we can
add additional states to support incremental sweeping and/or multi-
stage cell destruction.
