Develop List of 'System' Identifiers

We need a central list of function names that can be used with the System access modifier.

The prime example is `_x_alloc` which serves as the backend function for allocating dynamic memory.

Example usage:
```Haskell
system _x_alloc nBytes = ...

```
Once the function is defined, the system will know to use the provided functionality for dynamic memory allocation (e.g. in data types, lists, trees, etc.)

This list needs to be expanded to encompass all interactions between the system and language backends.