[dev-context] Register allocation handling

[Taco Hoekwater]

Hi,

I've just committed a new version to the repository that uses
omega-style (flat array) register allocation instead of
etex-style (sparse array).

This means that luatex now has 65536 (2^16) of each of the
registers

   \newcount
   \newskip
   \newmuskip
   \newdimen
   \newtoks
   \newbox
   \newmarks

The marks code is completely new, and not very optimized yet, but
it should not be much worse than pdfetex for a 'normal' document
(when no more than 50 or so separate mark classes).

Cheers, Taco