Start to get instance fields working.

test/field_use_before_set.wren

@@ -0,0 +1,9 @@
+class Foo {
+  write { io.write(_field) } // Compile a use of the field...
+  init { _field = "value" }  // ...before an assignment to it.
+}
+
+var foo = Foo.new
+// But invoke them in the right order.
+foo.init
+foo.write // expect: value