Faster Hash#slice (esp. when key count increases)

See title.

I claim no credit for this patch: credit goes to Grant Hollingworth (see http://merb.lighthouseapp.com/pr...). Just want to get it into Rails since it's trivial. I didn't replicate the Hash#except patch (until someone or I am able to profile memory usage).

Benchmark I used included. These are my results:

                     user     system      total        real
small hash#slice_current  1.210000   0.000000   1.210000 (  1.216810)
small hash#slice_patched  0.870000   0.000000   0.870000 (  0.871604)
                     user     system      total        real
big hash#slice_current  0.720000   0.000000   0.720000 (  0.717137)
big hash#slice_patched  0.020000   0.000000   0.020000 (  0.024483)