Even more array builtin methods

[am-cid]

1. flatten-one_d_array()
   • flattens the array to one dimension
   • if already one dimension, will do nothing
   • returns a one dimension variant of the current array type
2. join-senpai(separator-senpai)
   • use flatten() to flatten the array to one dimension, converts each element into senpai, then combines all elements with the provided separator