change util.randrange to bit-wise

[pik]

The current util.randrange runs up to worst case mean of 256 sys calls. This is, I think, the simplest way to implement bit-wise instead of byte-wise randrange. In terms of speed bit-wise is a lot more consistent (only where order is close to but less than (256**j) (It should be +2 but the original implementation calculated orderlen incorrectly) does it fall behind).

>>> timeit(randrange_old, order=generator_521.order(), n=10**5)
(0.0012479020285606384, 128.32433)  #mean time per run, mean os.urandom calls
>>> timeit(randrange_bit, order=generator_521.order(), n=10**5)
(6.894484043121337e-05, 1.0)   #mean time per run, mean os.urandom calls```

[warner]

did you mean len(bin(x)) here?

[pik]

@warner
You're right, I apologize, that was very careless.

[tomato42]

@pik Could you rebase on top of current master?