Combines an array of deques into one deque.

When glue is present, all of its elements are inserted between each adjacent pair of deques.

glue, and all of the deques in deqs, must share the same type. For example, if glue is "-", then deqs must be an array of strings.

(let words '("Strike" "the" "Earth!"))
(let sentence (join words " "))
(prn sentence) ; prints Strike the Earth!

If deqs is empty, returns an empty array (or an empty string if glue is a string).