message: das.named: Add logs.
diff: #include <utility>
  #include <elle/attribute.hh>
  +#include <elle/log.hh>
  #include <elle/meta.hh>
  #include <das/Symbol.hh>
  @@ -97,6 +98,9 @@ namespace das
  RTail&& ... remaining,
  Store&& ... store)
  {
  + ELLE_LOG_COMPONENT("das.named");
  + ELLE_DUMP(
  + "found named argument for %s: %s", RHead::name(), match.value);
  return next::apply(defaults,
  f,
  std::forward<Applied>(applied)...,
  @@ -183,6 +187,9 @@ namespace das
  StoreHead&& store_head,
  StoreTail&& ... store_tail)
  {
  + ELLE_LOG_COMPONENT("das.named");
  + ELLE_DUMP(
  + "use positional argument for %s: %s", Head::name(), store_head);
  return next::apply(
  defaults,
  f,
  @@ -228,6 +235,10 @@ namespace das
  Applied ... applied,
  Store&& ... store)
  {
  + ELLE_LOG_COMPONENT("das.named");
  + ELLE_DUMP(
  + "use default value for %s: %s",
  + Head::name(), defaults.default_type::value);
  return next::apply(
  defaults,
  f,
  @@ -331,6 +342,9 @@ namespace das
  List<>>::template result_of<F>::type
  call(F const& f, Args&& ... args) const
  {
  + ELLE_LOG_COMPONENT("das.named");
  + ELLE_TRACE_SCOPE("Prototype(%s): invoke %s%s",
  + this, f, std::tuple<Args const& ...>(args...));
  return Applier<
  DefaultStore,
  List<typename make_formal<Formal>::type...>,
  