utils: Change gcd function from recursive to interative

[honggyukim]

It'd be faster to use interative implementation than recursive one.

[BenWiederhake]

I agree that the iterative version could be faster, but apparently it's not relevant.

So let's test this. The worst-case clearly is large numbers, and occurs when the ratio between a and b is close to the golden ratio. (Intuition: if the ration is very large, then the first step reduces a and b very quickly because the modulo factor is so large. If the ratio is very small, then the modulo remainder cannot be big, so the first step again reduces a nad b very quickly.)
So let's use the numbers a = 18446744073709551615 = 2^64-1 and b = 11400714819323197569 ~= a/phi.

One can just compile this with the same options as tgl will be compiled, and run some experiments.

As you can see, it doesn't really make a difference, and actually looks like the recursive version is a bit faster, if at all. Also note that the stack will reach at most a depth of 93 = log_phi (2^64-1), so there really isn't much to optimize.

The long wait on startup is more likely due to bn_factorize at the bottom of the same file, which implements the proof-of-work part of the login process. I'd love to see an even better implementation of that.