Hide _S_n_primes from user code

[Jonathan Wakely]

On 22/04/15 22:11 +0200, François Dumont wrote:
>+    constexpr auto __n_primes
>+      = sizeof(__prime_list) / sizeof(unsigned long) - 1;

Normally I'd say

        sizeof(__prime_list) / sizeof(*__prime_list) - 1

would be better, but since it's very unlikely we'll change the element
type in the array what you have should be safe.

OK for trunk.