[graphds.h] Allocate graph from obstack

[Dimitrios Apostolou <jimis at gmx dot net>]

Initially I had one obstack per struct graph, which was better than using 
XNEW for every edge, but still obstack_init() called from new_graph() was 
too frequent.

So in this iteration of the patch the obstack is static global, 
initialised once and never freed. Please advise on whether this is 
acceptable, and also where I should initialise the obstack once, and avoid 
checking if it's NULL in every use.

Minor speed gains (couple of ms), tested with pre-C++ conversion snapshot, 
I'll retest soon and post update.


Thanks,
Dimitris

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