Allow Mojo structs as map keys

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

 // static
 {{struct.name}}Ptr {{struct.name}}::New() {
   {{struct.name}}Ptr rv;
   mojo::internal::StructHelper<{{struct.name}}>::Initialize(&rv);
   return rv;
 }
 
 {{struct.name}}::{{struct.name}}()
 {%- for field in struct.fields %}
     {% if loop.first %}:{% else %} {% endif %} {{field.name}}({{field|default_value}}){% if not loop.last %},{% endif %}
 {%- endfor %} {
 }
 
 {{struct.name}}::~{{struct.name}}() {
 }
+
+{%- if struct|is_hashable %}
+size_t {{struct.name}}::Hash(size_t seed) const {
+{%-   for field in struct.fields %}
+  seed = mojo::internal::Hash(seed, this->{{field.name}});
+{%-   endfor %}
+  return seed;
+}
+{%- endif %}