url-decode-url missing

[Juri Linkov <juri@linkov.net>]

There is 'url-encode-url' in lisp/url/url-util.el, but
no inverse function with the expected name 'url-decode-url'.

There is a pair of url-hexify-string/url-unhex-string,
and it seems url-unhex-string does something like decoding,
but its output is garbled text.  For example:

(let ((url "https://en.wiktionary.org/wiki/Ω"))
  (string-equal url (url-unhex-string (url-encode-url url))))
=> nil

Using decode-coding-string doesn't help either:

(let ((url "https://en.wiktionary.org/wiki/Ω"))
  (string-equal url (decode-coding-string
                     (url-unhex-string (url-encode-url url))
                     'utf-8)))
=> nil