Replaces `Rc<RefCell<Vec<Value>>>` with a new `GlobalStore` struct for
accessing global variables. This provides a more structured way to
manage global values and allows for future optimizations like sharing
immutable RTL portions across environments. The `GlobalStore` also
includes an `rtl_len` field to distinguish between RTL and user global
slots, enabling debug-time checks for writes to immutable RTL slots.
Brummel
0e8cd0832f