Handle inherited fields correctly.

test/inheritance/is.wren

@@ -0,0 +1,16 @@
+class A {}
+class B is A {}
+class C is B {}
+var a = A.new
+var b = B.new
+var c = C.new
+
+io.write(a is A) // expect: true
+io.write(a is B) // expect: false
+io.write(a is C) // expect: false
+io.write(b is A) // expect: true
+io.write(b is B) // expect: true
+io.write(b is C) // expect: false
+io.write(c is A) // expect: true
+io.write(c is B) // expect: true
+io.write(c is C) // expect: true