[bug#30657] Add support for file-like objects to the Prosody configuration

["Clément Lassieur" <clement@lassieur.org>]

Hello,

These patches add support for file-like objects to the Prosody service
configuration.  The idea is to replace this mechanism (which is used
very often):

    (plain-file
      (with-output-to-string
         ...
         (format #t ...)
         ...))

with this one:

    (mixed-text-file
      (flatten
        (with-tokens-to-list
           ...
           (push-tokens ...)
           ...))

The point is that tokens don't have to be strings.

Comments are welcome!
Clément