Added experimental support for integrity encoder & decoder.

Added experimental support for integrity encoder & decoder.
I'm going to rewrite things again.

* sasl.el (sasl-client-set-encoder): New function.
(sasl-client-set-decoder): New function.
(sasl-client-encoder): New function.
(sasl-client-decoder): New function.

* sasl-digest.el: Require 'cl' when compiling.
(sasl-digest-md5-signing-encode-magic): New constant.
(sasl-digest-md5-signing-decode-magic): New constant.
(sasl-digest-md5-htonl-string): New function.
(sasl-digest-md5-make-integrity-encoder): New function.
(sasl-digest-md5-make-integrity-decoder): New function.
(sasl-digest-md5-ha1): New function.
(sasl-digest-md5-response-value): Accept the 1st argument `ha1'.
(sasl-digest-md5-response): Use `sasl-digest-md5-ha1'.
- Set integrity encoder and decoder of the client.

* smtp.el: Require `luna'.
(smtp-read-response): Accept `smtp-connection' object rather than
process-object.
(smtp-send-command): Likewise.
(smtp-send-data): Likewise.