Function: primepi
Section: number_theoretical
C-Name: primepi
Prototype: G
Help: primepi(x): the prime counting function pi(x) = #{p <= x, p prime}.
Description:
 (gen):int        primepi($1)
Doc: the prime counting function. Returns the number of
 primes $p$, $p \leq x$. Uses a naive algorithm so that $x$ must be less than
 \kbd{primelimit}.
