[lltx] luaotfload update

[Khaled Hosny]

Hi all,

Few updates; the `unstable` branch has been merged into `master` and
further development will take place there, and there have been few more
syncs with ConTeXt and as the result we no longer patch any of original
files (thanks to Hans accepting few luaotfload specific patches), also
we now support byte code compiled font cache files (might do so for font
names database as well) which saves few milliseconds in font loading
(~2s when running `make check` on my machines) but firs time the font is
loaded is a bit slower (because of time used to compile the cache).

Now the main issue holding the new release is the removed tlig and trep
feature, I'm still not sure what to do here.

(BTW, LM and TeX Gyre fonts have this covered by `dlig` feature, which
is how it have always been done in TeX; a font feature implemented in
every individual font that wants it and not some input mapping or engine
feature.)

Regards,
 Khaled