create size size must be less than ((word size - 2) * max array length)
get t pos get the value in position pos, raises Invalid_argument if the position
is out of bounds.
set t pos set the value in position pos, raises Invalid_argument if the position
is out of bounds.
clear t set the contents of every element to false O(n / (word_size - 2))
fold t ~init ~f Fold over the array as in Array.fold
iter t ~f Iterate over the array as in Array.iter