improved JSON-LD context
renamed module interface to core
renamed Resource to Core in all concerned classes

IResource, HostedResource, HttpResource, ILocalResource, ILocalResource, StandaloneResource,
EditableResource, _DeletedResource
split rdflib.core into rdflib.cores and rdflib.wrappers
reorganized rdfrest modules

  utils.py -> util/__init__.py
  compat.py -> util/compat.py
  iso8601.py -> util/iso8601.py
  proxystore -> util/proxystore.py
  config -> util/config.py

  cores.py -> cores/__init__.py
  factory.py -> cores/factory.py
  hosted.py -> cores/hosted.py
  http_client.py -> cores/http_client.py
  local.py -> cores/local.py
  mixins.py -> cores/mixins.py

  serializers.py -> serializers/__init__.py
  serializers_html.py -> serializers/html.py
Merge branch 'align-with-salad' into develop