wrenReturn___() -> wrenSlotSet___().

This turns those functions into general-purpose functions for writing
raw C values into slots on the foreign call stack.

Writing a return just means writing a value to slot 0.

test/api/slots.h

@@ -0,0 +1,3 @@
+#include "wren.h"
+
+WrenForeignMethodFn slotsBindMethod(const char* signature);