mpz_jacobi

[Niels Möller]

I've pushed in the new mpz_jacobi now.

I also enabled the large quotient tests in t-jac.c, which makes testing
slower. I reduced the sizes used for testing a bit to make it more
tolerable. Feel free to tweak further if testing takes too much time
(currently t-jac takes 2.6 s on my x86_32 laptop and 1.1 s on x86_64
shell).

I wonder if nextprime and nextprime_step (in t-jac.c) shouldn't use a
larger prime table; the Handbook of Applied Cryptography recommends a
table of about 10000 small primes but I haven't done any relevant
benchmarks recently.

Regards,
/Niels

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