This includes a protocol for creating LibGC Heap allocated Swift
objects. Pay no attention to the Unmanaged shenanigans, they are
all behind the curtain.
This will allow us to use the GC to manage the lifetime of objects
that are not C++ objects, such as Swift objects. In the future we
could expand this cursed FFI to other languages as well.
While we don't want arbitrary callers deferring GC, we do want
deferral to be available to the Swift. In order for Swift to
understand the RAII nature of DeferGC, we need to mark it as
non-copyable.
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
