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 :^)
Empty string is extremely common and we can avoid a lot of heap churn
by simply caching one in the VM. Primitive strings are immutable anyway
so there is no observable behavior change outside of fewer collections.
More steps towards multiple global object support. Primitive cells
like strings, bigints, etc, don't actually have any connection to
the global object. Use the explicit API to clarify this.
To make sure that everything is set up correctly in objects before we
start adding properties to them, we split cell allocation into 3 steps:
1. Allocate a cell of appropriate size from the Heap
2. Call the C++ constructor on the cell
3. Call initialize() on the constructed object
The job of initialize() is to define all the initial properties.
Doing it in a second pass guarantees that the Object has a valid Shape
and can find its own GlobalObject.
