Fix up how enums work. Now they're always strings

examples/addressbook.capnp

@@ -0,0 +1,32 @@
+@0x934efea7f017fff0;
+
+struct Person {
+  id @0 :UInt32;
+  name @1 :Text;
+  email @2 :Text;
+  phones @3 :List(PhoneNumber);
+
+  struct PhoneNumber {
+    number @0 :Text;
+    type @1 :Type;
+
+    enum Type {
+      mobile @0;
+      home @1;
+      work @2;
+    }
+  }
+
+  employment @4 union {
+    unemployed @5 :Void;
+    employer @6 :Text;
+    school @7 :Text;
+    selfEmployed @8 :Void;
+    # We assume that a person is only one of these.
+  }
+}
+
+struct AddressBook {
+  people @0 :List(Person);
+}
+