Every Promise allocates two PromiseResolvingFunction cells (resolve and
reject). Each cell stored an AK::Function holding a closure, but neither
closure captured anything beyond the Promise and AlreadyResolved already
present as cell fields.
Hoist the two closure bodies into static functions on Promise and
dispatch by a Kind enum on the cell. This removes the AK::Function
storage embedded in each cell along with its off-GC malloc'd closure
capture.
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.
