add import helper, new directory layout, revised letterspacing, fontloader gone nuts

* The fontloader now occupies its own subtree in ``src/``.
* Files actually required by Luaotfload at runtime are kept in a
  tree separate from the merged imports.
* Import helper for the lazy mainainer.
* **Hans synced the fontloader code with the revised one in Context,**,
  bringing upstream on par with the last year or so of Luatex development.
* As a consequence of the last point, we are free to use ``node.direct()``
  wherever we please, so  I converted the letterspacing callback to the new
  API.