[ELISP] How do you turn an array of chars into a string?

[Joseph Brenner]

The elisp manual has this example, using "kbd" to convert a (relatively)
readable string into the "internal Emacs key representation":

     (global-set-key (kbd "C-x C-\\") 'next-line)

     (global-set-key [?\C-x ?\C-\\] 'next-line)

What's the inverse of kbd?  What if you want to convert an array-of-chars
into a string?

Things like this seem to work, but only for very simple chars:

  (mapconcat 'string [?c ?a ?t] "")  ;; => "cat"

Motivation: see the first FIXME in footnote.el, on footnote-prefix.