`xso::generator` — Write Access to the State

We have methods that set the state in various ways:

1void seed();

2constexpr void seed(word_type seed);

template<typename Iter>
3constexpr void seed(Iter b, Iter e);

1: Sets the state primarily from a std::random_device.
2: Sets the state quickly and repeatably from a single unsigned seed integer.
3: Sets the state by copying all the words from an iteration.

Notes:

The term “seeding” is very commonly used in the context of random number generation.
The no-argument version of seed sets the underlying state fully from a default source of entropy by multiple calls to std::random_device. Reportedly, some implementations of that standard facility are not great, so we also mix in a scrambled call to a high-resolution clock for one of the state words.
Seeding generators with multiple words of state from a single word is never ideal.
Nevertheless, it is often handy to be able to do exactly this when you are trying to prototype a simulation of some sort. For that stage of development, being able to run and rerun things with an easily constructed “fixed” random number stream is very useful.
To mitigate some of the worst features of this type of construction, the implementation uses a scrambled version of the one input word as a seed to a simple, small state but pretty effective SplitMix64 random number generator. That generator then creates a full state array for the xoshiro/xoshiro in question.
If you do construct a generator by passing in an initial state array then be sure that the values are not all zero as that is a fixed point for these generators.