Implement the RNGScope class for RAII-based RNG control.

[LTLA]

Inspired by the Rcpp class of the same name. Usage is demonstrated in examples/ex_rng_scope.cpp; it enables people to easily set the seed for reproducible execution of various RNG-dependent igraph functions. For example:

{
     ig::RNGScope scope(1000); // sets PCG32 with seed 1000.
     // do something with igraph functions that might use the RNG.
} // previous RNG is restored upon exiting the scope.

Multiple RNGScopes can be safely nested within each other's lifetimes, so users don't have to worry about whether they are interfering with an igraph RNG defined elsewhere if they want reproducible execution of their own code.

void foo() {
    ig::RNGScope scope1(2000, &igraph_rng_mt19937); // sets MT with seed 2000
    // do something with igraph functions that might use the RNG.
}

int main() {
    ig::RNGScope scope2; // uses PCG32 with the default seed
    // do something with igraph functions that might use the RNG

    foo(); // uses 'scope1' internally but sets it back to 'scope2' on exit.

    // do more things with the 'scope2' RNG.
    return 0;
}

[szhorvat]

Can you please use RNG_INTEGER and the similar macros in all places in the example?

[szhorvat]

I am not sure I'm 100% happy with the details of this yet, but everything's in flux at the moment anyway. I'll comment more when I get the chance. I'm merging now in the interest of progress and testability. Polishing is possible later.