The old wrenReturn___() functions only wrote to slot 0 for return values. The new wrenSetSlot___() functions allow writing to any slot index, making them general-purpose for manipulating the foreign call stack. This change updates all call sites, removes the dedicated returns test suite in favor of slots tests, and adjusts the VM internals to treat slot 0 as the return position rather than using a separate return mechanism.
Migrate the foreign method argument access API from an argument-oriented naming
scheme to a slot-based one, renaming wrenGetArgumentCount to wrenGetSlotCount,
wrenGetArgumentBool to wrenGetSlotBool, and all other argument accessors
accordingly. Update every call site across the VM, modules (io, timer, meta,
random), and test files (benchmark, foreign_class) to use the new names. The
slot getters now assert the correct type at runtime rather than silently
returning defaults, shifting safety responsibility to the caller for
performance. Also add the IS_LIST type-checking macro to wren_value.h alongside
the existing IS_* macros.
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.
