sync with Context, wipe out more compatibility code

* this time, the sync entails rather drastic changes, reflecting [Hans’s recent work on the fontloader](http://repo.or.cz/w/context.git/shortlog/refs/heads/origin) (some recent changes were retracted, the fontloader moved from the recently introduced *nuts* (``node.direct.*``) to a more stable base)
* new files: ``luaotfload-fonts-inj.lua`` as well as ``luaotfload-fonts-otn.lua`` (replacing packaged ``font-otn.lua`` and ``font-inj.lua`` from Context)
* remove further compatibility code from ``luaotfload-auxiliary.lua`` (not done yet, there are some redundant *tfmdata* lookups that should be dropped)
* small  fixes