Inserts one or more elements into a deque.
The index can be negative, or just past the end of the deque. The left-most inserted
element will have the index idx
.
(let ar (arr 'a 'b 'c))
(insert! ar 1 'x 'y)
(prn ar) ; prints (a x y b c)
(insert! ar 5 'd)
(prn ar) ; prints (a x y b c d)