The number of arguments can be derived from the stack pointer difference, eliminating the need to store it separately. Rename the slot pointer to better reflect its purpose as the start of the foreign call's stack region.
Rename the `src/aux/` directory to `src/optional/`, update all include guards, preprocessor defines, and file references from `WREN_AUX_*` to `WREN_OPT_*`, and adjust build system, Xcode project, and code generation scripts accordingly.