Unroll pkgCache::sHash 8 time, break up dependency

Unroll pkgCache::sHash 8 times and break up the dependency between
the iterations by expanding the calculation
    H(n) = 33 * H(n-1) + c
8 times rather than performing it 8 times. This seems to yield about
a 0.4% performance improvement.

I tried unrolling 4 and 2 bytes as well, those only having 3 ifs at
the end rather than 1 small loop; but that was actually slower -
potentially the code got to large and the cache went bonkers.

I also tried unrolling 4 times instead of 8, thinking that smaller
code might yield better results overall then, but that was slower as
well.

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