Fix hangs in binomial sampling

[dhardy]

As noted here it appears that I merged #735 too soon, and it has been causing some tests to hang, though apparently only on MIPS.

[dhardy]

Progress report:

• after an argument with SELinux I am able to run the MIPS test locally
• putting a limit on the loop for the small-expected-value of 100 iterations, the test succeeds on the native target but hits this limit when run on MIPS

On MIPS, it fails with this output (from just before the loop starts and panicking when x>100):

Sampling from Bin(20, 0.7)
Start: u=0.9136724307867923, r=0.0000459986536544739, a=9.000000000000002, s=0.42857142857142866, q=0.7
thread 'main' panicked at 'Too many tries! u=0.8560244207867922, r=0, x=101', src/distributions/binomial.rs:115:21

Here r is wrong. Is this because of the pow method?

Edit: using let mut r = q.powi(self.n as i32); instead appears to fix the test (no hang), so this would appear to be the case, though the problem does not appear to be with bit logic (using numeric operations instead does not help).

[vks]

Could this be due to different rounding behavior?

[dhardy]

I don't know. It seems quite a large error, for I think 5 FP ops.