function
boot.janet on line 1570, column 1
(flatten-into into xs)
Takes a nested array (tree), and appends the depth first traversal
of that array to an array 'into'. Returns array into.
(flatten-into @[1 2 3] @[[4 5 6] @[7 8 9] @[10 11 12]]) # => @[1 2 3 4 5 6 7 8 9 10 11 12]