Add :class:`importlib.abc.ResourceReader` as an ABC to provide a
unified API for reading resources contained within packages. Loaders
wishing to support resource reading are expected to implement the
``get_resource_reader(fullname)`` method.

Also add :mod:`importlib.resources` as the stdlib port of the
``importlib_resources`` PyPI package. The modules provides a high-level
API for end-users to read resources in a nicer fashion than having to
directly interact with low-level details such as loaders.

Thanks to this work, :class:`importlib.abc.ResourceLoader` has now
been documented as deprecated due to its under-specified nature and
lack of features as provided by :class:`importlib.abc.ResourceReader`.