Convert get_pieces to array

[abrahimt]

          I know I wrote this function, but we should change this to an array since we know it's always going to be 12 long.

(Can you store tuples inside arrays?!?!) Prefer stack over heap.

_Originally posted by @chanson02 in https://github.com/chanson02/KnO3/pull/42#discussion_r1401119843_