Handle inherited fields correctly.

test/inheritance/inherit_fields.wren

@@ -0,0 +1,35 @@
+class Foo {
+  foo(a, b) {
+    _field1 = a
+    _field2 = b
+  }
+
+  fooPrint {
+    io.write(_field1)
+    io.write(_field2)
+  }
+}
+
+class Bar is Foo {
+  bar(a, b) {
+    _field1 = a
+    _field2 = b
+  }
+
+  barPrint {
+    io.write(_field1)
+    io.write(_field2)
+  }
+}
+
+var bar = Bar.new
+bar.foo("foo 1", "foo 2")
+bar.bar("bar 1", "bar 2")
+
+bar.fooPrint
+// expect: foo 1
+// expect: foo 2
+
+bar.barPrint
+// expect: bar 1
+// expect: bar 2