Emit toString functions for all types.

* toString() is mainly for debugging purposes only.

* For HIDL internal types (hidl_string, hidl_vec, etc.)
  toString() is found in ::android::hardware::details.

* For a user defined type
  android.hardware.foo@1.0::IFoo.Type,
  toString() is found in ::android::hardware::foo::V1_0.

* For bitfield<::anroid::hardware::foo::V1_0::T>
  that gets translated to the underlying
  numeric type of T, it doesn't make sense to override toString().
  A templated toString() function for each user-defined HIDL enum \
  is introduced into the same namespace; call it with
    using namespace ::android::hardware::foo::V1_0;
    toString<IFoo::MyEnumType>(value);

Test: hidl_test and look at the output of logcat

Bug: 33459772

Change-Id: I70eee018e31d700bf1376334276dbd343af5615f
diff --git a/Interface.h b/Interface.h
index 0182818..87a50bc 100644
--- a/Interface.h
+++ b/Interface.h
@@ -89,6 +89,10 @@
             bool isReader,
             ErrorMode mode) const override;
 
+    status_t emitGlobalTypeDeclarations(Formatter &out) const override;
+    status_t emitTypeDefinitions(
+            Formatter &out, const std::string prefix) const override;
+
     void emitJavaReaderWriter(
             Formatter &out,
             const std::string &parcelObj,