experimental new dump

[Luiz Henrique de Figueiredo <lhf@...>]

I've written a version of ldump.c that saves only a single instance of any
string (of course, I've also written the corresponding lundump.c):
	http://www.tecgraf.puc-rio.br/~lhf/tmp/dump.tar.gz

This change should make precompiled chunks smaller and probably faster
to load. I'd like feedback on this change: if you have reasonably large
Lua programs for which you may be able to measure any difference in size
and time to load them in precompiled form, I'd appreciate hearing from
you. To try it, just replace the original ldump.c lundump.c lundump.h
with the ones in the tarball above and rebuild.

All feedback is welcome. Thanks.
--lhf