refactor: move global variable storage from VM into Module struct

Functions now hold a reference to the module where they were defined and load module-level variables from there instead of from the VM's global table. This introduces a new Module struct with its own variable buffer and symbol table, replacing the VM-level globals and globalNames. The main module is stored directly in the VM as a temporary measure. Performance regresses slightly due to the indirection through the function's module pointer.

include/wren.h

@@ -93,6 +93,8 @@ void wrenFreeVM(WrenVM* vm);
 WrenInterpretResult wrenInterpret(WrenVM* vm, const char* sourcePath,
                                   const char* source);
 
+// TODO: Figure out how these interact with modules.
+
 // Defines a foreign method implemented by the host application. Looks for a
 // global class named [className] to bind the method to. If not found, it will
 // be created automatically.