C++: Replace custom deleters with std::default_delete

C++: Replace custom deleters with std::default_delete

Replace custom Deleter classes with std::default_delete<>, declared
as friend so it can invoke the private destructor. Inexplicably,
std::shared_ptr<> does not use default_delete<> by default, so it
is still necessary to explicitly specify the deleter when creating
shared_ptr instances.

With this, unique_ptr and shared_ptr instances now use the same
default delete mechanism.