Add state export for `rand_xoshiro`

[benruijl]

At the moment, it is not possible to export the state without using serde. Could you add a function that exports a [u8; 32] so that it can be loaded with from_seed?

[dhardy]

I see no issue with an addition fn state(&self) -> [_; _].

Could you [...]

This project operates on community contributions, though I'd also be happy with financial contribution.