- Move NodeMarker to its own file, and introduce a non
  templatized base class.
- Cleanup the include hell.
- Sanitize the Node construction methods now that we
  got rid of that GenericNode/GenericGraph stuff.
- Protect against NodeId overflow in Graph.
- Various minor cleanups.

TEST=cctest,mjsunit,unittests

Review URL: https://codereview.chromium.org/838783002

Cr-Commit-Position: refs/heads/master@{#25977}