Maintain a HashTable<HeapBlock*> of live heap blocks in the Heap,
updated on block creation and destruction.
Weak containers (WeakMap, WeakSet, WeakRef, FinalizationRegistry)
now check block liveness before accessing cell memory in their
remove_dead_cells() methods. This prevents use-after-free when
blocks have been freed during incremental sweeping.
Move weak container cleanup (remove_dead_cells) out of both
sweep_dead_cells() and start_incremental_sweep() to the place
where it is actually safe to inspect cell state: collect_garbage().
Previously, remove_dead_cells could access cells that had already
been swept and poisoned by ASAN, causing use-after-poison crashes
when a new GC triggered while an incremental sweep was in progress.
It's harmless if the WeakRef is asked to remove dead cells when it has
no dead cells to remove. This can happen if the WeakRef lives longer
due to someone referencing it. Caught by test-js with different GC
frequency.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.
As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.
