Make math_dumb use p_3 sieve; write pi-sq

I saw while reworking my paper that a p_3 sieve instead of the p_5 one used
up until now yields more than twice speed and only 5% less space savings. I
regenerated the pictures and stuff - the bench3 is made prettier now anyway.

NAIVE(pi)() is an alias for NAIVE(get_n_below_or_equal)() as remarked in the
paper.

The pi-sq program (formerly mistakenly called lambda-prime-any) can compute
pi(N) with a pt with primes up to sqrt(N). It contains the most optimised
version of a Lambda' sum function I can think of - using the pt whenever
possible to lookup values.