Improve speed of prime generation especially for RSA keygen
Improve speed of prime generation especially for RSA keygen

In RSA keygen we have to verify that p-1 and e are coprime. But this
is expensive to compute. So first do a single round of Miller-Rabin
primality test; only if that passes do we test coprimality. Improves
RSA keygen times notably. All times averaged over many keygens:

1024-bit  21.74 ms ->  10.78 ms
2048-bit  94.93 ms ->  62.80 ms
3072-bit 296.79 ms -> 198.12 ms
4096-bit 738.07 ms -> 499.10 ms

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