factor: simplify primes table

factor: simplify primes table

* src/factor.c (primes_ptab): New table of primes, replacing
primes_diff and primes_diff8.  All uses changed.  This is simpler
and should improve performance slightly.  Although this limits the
table’s primes to 2**15 instead of to 668221, the limit can easily
grow to 2**32 by changing the type of ‘prime’, without hurting
performance significantly compared to the primes_diff and
primes_diff8 approach.
* src/make-prime-list.c (output_primes):
For each prime p, output p instead of two differences.