Reseeding Xoroshiro128Plus not equivalent to creating a new one

MWE (using RandomNumbers 1.6.0)

```julia
julia> using RandomNumbers

julia> seed = 12
12

julia> rng = Xorshifts.Xoroshiro128Plus(12)
RandomNumbers.Xorshifts.Xoroshiro128Plus(0x591545ee404ceb86, 0xe9c6c533699b4284)

julia> Random.seed!(rng, 12)
RandomNumbers.Xorshifts.Xoroshiro128Plus(0x018c018c0000000c, 0x8180018000003001)
```

So, I may be misunderstanding what should happen here, but from the `Random.seed!` doc string I would have expected the two objects to be the same.

Start by reproducing the Julia MWE with Xoroshiro128Plus and compare the constructor with Random.seed! for the same seed. Trace the relevant entry points and add coverage showing the intended equivalence; done means reseeding an existing generator produces the same state as constructing a new one.

Scritto dal modello di indicizzazione a partire dal testo della issue.