Fix a bug in Miller-Rabin primality testing introduced in 1.8.3
where we chose a single random nonce and tested it repeatedly, rather
than choosing new nonces each time. Reported by Jeff Marrison.

Also remove a pointless comparison (also pointed out by Jeff), add an
initial test using a witness of 2, and increase the random nonces from
64 to 128 bits.
Release 1.10.8