Allow Mojo structs as map keys

mojo/public/tools/bindings/generators/cpp_templates/struct_serialization_declaration.tmpl

 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- set data_view = struct|get_qualified_name_for_kind ~ "DataView" %}
 {%- set data_type = struct|get_qualified_name_for_kind(internal=True) %}
 
 namespace internal {
 
 template <typename MaybeConstUserType>
 struct Serializer<{{data_view}}, MaybeConstUserType> {
   using UserType = typename std::remove_const<MaybeConstUserType>::type;
   using Traits = StructTraits<{{data_view}}, UserType>;
@@ skipping 19 matching lines @@
     if (CallIsNullIfExists<Traits>(input)) {
       *output = nullptr;
       return;
     }
 
     void* custom_context = CustomContextHelper<Traits>::GetNext(context);
 
     {{struct_macros.serialize(
           struct, struct.name ~ " struct",
           "CallWithContext(Traits::%s, input, custom_context)", "result",
-          "buffer", "context", True)|indent(4)}}
+          "buffer", "context", True)|indent(2)}}
     *output = result;
 
     CustomContextHelper<Traits>::TearDown(input, custom_context);
   }
 
   static bool Deserialize({{data_type}}* input,
                           UserType* output,
                           SerializationContext* context) {
     if (!input)
       return CallSetToNullIfExists<Traits>(output);
 
     {{data_view}} data_view(input, context);
     return Traits::Read(data_view, output);
   }
 };
 
 }  // namespace internal