• RelayNetworkDebug now logs query variables.
• RelayNetworkDebug is now added as a subscriber instead of replacing any
  existing network layers. It also no longer replaces the global fetch.
• Relay.Environment#injectNetworkLayer (and, by extension
  Relay#injectNetworkLayer) will now warn if injecting would overwrite a
  previously injected layer.
• Added experimental low-level RelayGraphQLMutation API (still being
  finalized and not yet documented, so use at your own risk).