Faster AA sampling #350

We might be able to randomly sample 8 AAs at a time by sampling from 0:20^8 - 1, and then computing each base-20 digit in parallel, before reinterpret'ing to UInt64.
Something like

function foo(rng)
    x = rand(rng, UInt(0):UInt(20)^8-1)
    a = div(x, 20^0) % 20^1
    b = div(x, 20^1) & 20^2
   .....
   return reinterpret(UInt64, (a, b, c, ... h))
end

Also benchmark compared to just a normal divrem chain (which LLM may reorder), and also benchmark compared to status quo