This cache is referenced by a few parts of the JS API spec, including
the threads spec (such as in toFixedLengthBuffer), as well as the
"refresh the Memory buffer" algorithm, which was implemented as a method
of Memory before this change.
Now, this algorithm can be implemented in a spec-like fashion (though it
mostly seems to add extra complexity). This change also fixes a bug
where memories that were re-exported from an imported WebAssembly.Memory
were given a distinct WebAssembly.Memory object, since the caching that
existed in Instance.cpp was instance-local, not global to the realm.
We also make Memory::m_buffer non-lazy, since we have to implement
"initialize a memory object" correctly anyway.
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
