Faster sha256 function

[TrainerRed151]

Calling hashlib.sha256 directly with the hexdigest() output is an order of magnitude faster.

Before change:

>>> timeit.timeit("import cryptos; cryptos.sha256('foo')", number=1000)
0.01851644799999974

After change:

>>> timeit.timeit("import cryptos; cryptos.sha256('foo')", number=1000)
0.0015009069999996072

[char23-web]

trainerred151:master