Method shared_string_ref

include/libremidi/system_error2.hpp:1439–1447

  }

  explicit shared_string_ref(string_arena* a) noexcept
      :
      string_ref{
          a->begin(),
          a->end(),
          string_ref::resource_management{&copy_construct, &move_construct, &destroy},
          a
      }
  { }

  const string_arena_base* get_arena() const noexcept
  {