sha-256 perf

[cmazakas]

First pass attempt at speeding up the sha-256 implementation.

On my machine, benchmark/buffer.cpp is:

boost::hash2::sha2_256 (N=65536): 174029441: 14975 ms, 273.52 MB/s
--
boost::hash2::sha2_256 (N=15): 174029441: 15397 ms, 266.02 MB/s
--
boost::hash2::sha2_256 (N=4): 174029441: 18116 ms, 226.10 MB/s

and with these changes:

boost::hash2::sha2_256 (N=65536): 174029441: 12777 ms, 320.57 MB/s
--
boost::hash2::sha2_256 (N=15): 174029441: 13570 ms, 301.84 MB/s
--
boost::hash2::sha2_256 (N=4): 174029441: 15982 ms, 256.28 MB/s

[pdimov]

+17% isn't bad at all.