refactor: replace wrenReturn___() with wrenSetSlot___() for writing values to arbitrary slots

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.

src/optional/wren_opt_meta.c

@@ -19,12 +19,10 @@ void metaCompile(WrenVM* vm)
 
   // Compile it.
   ObjFn* fn = wrenCompile(vm, module, wrenGetSlotString(vm, 1), false);
-  if (fn == NULL) return;
 
   // Return the result. We can't use the public API for this since we have a
   // bare ObjFn.
-  *vm->foreignStackStart = OBJ_VAL(fn);
-  vm->foreignStackStart = NULL;
+  vm->foreignStackStart[0] = fn != NULL ? OBJ_VAL(fn) : NULL_VAL;
 }
 
 static WrenForeignMethodFn bindMetaForeignMethods(WrenVM* vm,

src/optional/wren_opt_random.c

@@ -92,14 +92,14 @@ static void randomFloat(WrenVM* vm)
   // from 0 to 1.0 (half-inclusive).
   result /= 9007199254740992.0;
 
-  wrenReturnDouble(vm, result);
+  wrenSetSlotDouble(vm, 0, result);
 }
 
 static void randomInt0(WrenVM* vm)
 {
   Well512* well = (Well512*)wrenGetSlotForeign(vm, 0);
 
-  wrenReturnDouble(vm, (double)advanceState(well));
+  wrenSetSlotDouble(vm, 0, (double)advanceState(well));
 }
 
 // TODO: The way these are wired up is pretty verbose and tedious. Also, the