rand/xoshiro: use a std::array to store the state

This makes it marginally easier to work use with std algorithms

rand/xoshiro.hpp

@@ -19,6 +19,7 @@
 
 #include "../std.hpp"
 
+#include <array>
 #include <limits>
 #include <random>
 
@@ -48,6 +49,6 @@ namespace cruft::rand {
         void long_jump (void);
 
     private:
-        u64 m_state[4];
+        std::array<u64, 4> m_state;
     };
 }