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.wren

@@ -0,0 +1,16 @@
+class Slots {
+  foreign static noSet
+
+  foreign static getSlots(bool, num, string, bytes, value)
+
+  foreign static setSlots(a, b, c, d)
+}
+
+// If nothing is set in the return slot, it retains its previous value, the
+// receiver.
+System.print(Slots.noSet == Slots) // expect: true
+
+var value = ["value"]
+System.print(Slots.getSlots(true, "by\0te", 12.34, "str", value) == value) // expect: true
+
+System.print(Slots.setSlots(value, 0, 0, 0) == value) // expect: true