Develop Backend Framework for Access Control

It is not immediately clear where this should belong, but access control needs to be integrated into the current functions system.

It may be sufficient to add an accessor enum structure to [FunctionNode's Protected Variables](https://github.com/CodeusTech/Xita-AArch64/blob/PreAlpha-Dev/src/xcs/expressions/functions/functions.h#L35).

To make private access work, the Nodes' Module `_context` id number will need to be made public so it can be checked at compile-time for allow-ability in use-cases.