file_io: added is_open() to FileReader and FileWriter.
hash: doc tidy.
hash: added constexpr variants to calc functions.
resource: added type & name hash literals & functions.
memory: made Allocator::deallocate() take pointer-to-const.¹

1: Destructor can be called on a const object (as done in
TOGO_DESTROY()), and there's no reason for the user to expect an
allocator to mutate the object directly on deallocation.