Add API to get the type of object in a slot.

test/api/slots.wren

@@ -2,10 +2,15 @@ class Slots {
   foreign static noSet
   foreign static getSlots(bool, num, string, bytes, value)
   foreign static setSlots(a, b, c, d)
+  foreign static slotTypes(bool, foreignObj, list, nullObj, num, string, unknown)
   foreign static ensure()
   foreign static ensureOutsideForeign()
 }
 
+foreign class ForeignType {
+  construct new() {}
+}
+
 // If nothing is set in the return slot, it retains its previous value, the
 // receiver.
 System.print(Slots.noSet == Slots) // expect: true
@@ -17,6 +22,9 @@ System.print(Slots.getSlots(true, "by\0te", 12.34, "str", value) == value)
 System.print(Slots.setSlots(value, 0, 0, 0) == value)
 // expect: true
 
+System.print(Slots.slotTypes(false, ForeignType.new(), [], null, 1.2, "str", 1..2))
+// expect: true
+
 System.print(Slots.ensure())
 // expect: 1 -> 20 (190)