divrep/prodroot: fixup - p<n> must translate to d<n>
divrep: switch Pari to GMP
divrep: allow command-line control of test order

Support -ts5,4,3,2,1 to control some or all of the test order
A249064 (and A090252): first pass

There are the lexically first sequences of positive integers such that
each element a(n) is coprime to the next a(n) (resp. n) elements.
A249064: refactor main loop into smaller functions
A249064: can't have free values without free factors
A249064: track small and large free primes separately

Most of the time all the free prime factors are larger than the
square root of the next unused prime, in which case there's no chance
of finding a product of those prime factors less than the next unused
prime. Tracking them separately allows a substantial speedup.
A249064: switch Math::Pari to Math::Prime::Util::GMP
A249064: show more useful output
A249064: specific checks when there's one free small prime

We know in this case that we need the minimum of nextp, p^k or p^k . q_i
that hasn't yet been seen, where p is the free small prime and q_i are
the available large primes. These avoids the full scan in almost all cases.