[patch] Fix of the CPU bottleneck in pack_vector

[Yuri]

Hi bison maintainers,

The attached patch fixes the CPU problem that these lines in pack_vector 
function caused in certain testcases:
      for (k = 0; ok && k < vector; k++)
        if (pos[k] == j)
          ok = false;

In the version 2.5, pos array essentially represents the integer set 
with unique integers placed into it in the random order.
In the above code section, pos is linearly scrolled through every time 
some j is tested for the set membership, and this was 98.5% CPU in 
certain testcases.

What this patch does:
it replaces pos integer array with pos_set boolean array. pos_set 
represents the same integer set as pos did, but in a way of membership 
bit indexed by the integer.
Such that set insertion and membership testing operations are done in 
one quick step at the expense of a little more memory allocated. Before 
insertion was quick, but membership testing was slow and caused the CPU 
bottleneck.

I believe this patch will speed any sizable test case up.

Please let me know if you find any problem with the patch.

Thanks,
Yuri

patch.txt
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