[ANN] guile-mqtt-0.2.0 released

[Mikael Djurfeldt <mikael@djurfeldt.com>]

Guile MQTT provides bindings for the libmosquitto MQTT client
library. The bindings are written in
[GOOPS](https://www.gnu.org/software/guile/manual/html_node/GOOPS.html)
and rely on lower-level bindings created by NYACC directly and
automatically from mosquitto.h.

Changes compared to v0.1.0:

* All functionality of Chicken Scheme mosquitto bindings now
  implemented and documented.

* Distribute own copy of (... foreign cdata). NYACC is now not
  required if building from a tar archive (as opposed to building from
  a git repository clone).

* Support make in separate build directory.

* Bugfix: unsubscribe now works.

The bindings align with GOOPS style, which means short method
names. (The specialization is done through the arguments.)

The user can extend the client class by inheritance.

Guile MQTT is free software and is released under the LGPL v3 license; the
full source distribution is available through

* a tarball:

https://github.com/mdjurfeldt/guile-mqtt/releases/download/v0.2.0/guile-mqtt-0.2.0.tar.gz

* the git repository:
     https://github.com/mdjurfeldt/guile-mqtt

Guile MQTT maturity is beta level.

The following example, as well as the Guile libmosquitto bindings
themselves,
are inspired by the [Chicken Scheme mosquitto
bindings](http://wiki.call-cc.org/eggref/5/mosquitto) by Dmitrii
Kosenkov.

```
(use-modules (mosquitto client))

(let ((client (make-client #:on-connect
                          (lambda (client err)
                            (if (not (eq? err MOSQ_ERR_SUCCESS))
                                (abort err)
                                (display "Yay, we are connected!"))))))
  (set! (disconnect-callback client)
        (lambda (client err)
          (if (not (eq? err MOSQ_ERR_SUCCESS))
            (display "Unexpected disconnect..."))))

  (set! (message-callback client)
        (lambda (cl msg)
          (display (string->append "Topic: " (topic msg)
                                   "Payload:" payload msg))
          (publish client "topic2" "message received, thanks!")))
  (connect client "localhost" #:username "mqtt-admin" #:password "mypass")
  (subscribe client "topic1")
  (loop-forever client))
```

See further examples under the directory [examples](
https://github.com/mdjurfeldt/guile-mqqt/tree/main/examples).