New power of 2 hash policy

[François Dumont]

Hi

    Here is a patch to offer an alternative hash policy. This one is
using power of 2 number of buckets allowing a faster modulo operation.
This is obvious when running the performance test that I have adapted to
use this alternative policy. Something between current implementation
and the tr1 one, the old std one.

    Of course with this hash policy the lower bits of the hash code are
more important. For pointers it would require to change the std::hash
implementation to remove the lower 0 bits like in the patch I proposed
some weeks ago.

    What do you think ?

François
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