Rewrite PRNG#15

The underlying algorithm is fine (AFAIK), but - munit_rand_uint32 is not composable but I use it like it is. Functions like rand_double, rand_memory, etc., should get a local copy of the state, generate as many random numbers as they need, then CAS the new state back to the global variable. - Expose munit_rand_uint32 instead of munit_rand_int. The latter could vary in size across different platforms, yielding different results. rand_int_range should be safe as long as min and max are not huge (outside of [INT32_MIN, INT32_MAX]) - Sticking to 32 bits means we could use something like https://stackoverflow.com/questions/2509679/how-to-generate-a-random-number-from-within-a-range#6852396 for uniform distribution.