remove sha256.c in favour of sha256_fast.c as it was shown to be faster.

65536 iterations took 0.413442 seconds.
eb c3 b6 66 ea 72 89 fa 86 c6 21 06 35 96 34 d2 18 6d a6 9d 98 3d e8 68 d3 51 d0 a0 02 f6 c8 8d
65536 iterations took 0.450461 seconds.
0a f2 f4 1c 60 58 27 da 4a 0f 7f 19 18 7e 83 f8 fc c5 9c 5d 88 97 71 7f 00 8e db 4c 8c bc 13 9d
modified timing tests (have not added hmac_sha256 timer yet) and added sha256 test vectors