Make result_type typedef in pcg_extras::seed_seq_from public

[timo-eichhorn]

Unless I'm somehow misunderstanding how pcg_extras::seed_seq_from is supposed to be used, I believe the result_type typedef in pcg_extras::seed_seq_from should be public instead of private. Otherwise, when trying to use pcg_extras::seed_seq_from with standard library PRNGs (say std::mt19937), compilation fails (for both gcc 12.1.0 and clang 14.0.0) due to the aforementioned typedef being private.

A simple example demonstrating the problem:

#include <random>
#include "PCG/pcg_random.hpp"
#include "PCG/pcg_extras.hpp"

int main()
{
    pcg_extras::seed_seq_from<std::random_device> seed_source;

    pcg64        pcg_generator(seed_source); // This works
    std::mt19937 mt_generator(seed_source);  // This does not
    return 0;
}

After the change, the program compiles without problems.