By moving the LHS and RHS pointers used by rope strings into a
RopeString subclass, we shrink PrimitiveString by 16 bytes. Most strings
are not rope strings, so this ends up saving quite a bit of memory.
PrimitiveString is now internally either UTF-8, UTF-16, or both.
We no longer convert them to/from ByteString anywhere, nor does VM have
a ByteString cache.
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
More work on decoupling the general runtime from Interpreter. The goal
is becoming clearer. Interpreter should be one possible way to execute
code inside a VM. In the future we might have other ways :^)
