bug#17636: Implement SCRAM-SHA-1 SASL mechanism

[Magnus Henoch <magnus.henoch@gmail.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>>> SCRAM-SHA-1 requires an implementation of HMAC-SHA1, so I added a module
>>>> for that, too.
>>> Do we really need a whole file for those three lines?
>> Good point.  Should I just put it in hmac-md5.el?  I can't think of any
>> more logical place for it.
>
> If I could choose, I'd merge all hmac-*.el into a single file.

I just discovered that there is a second HMAC implementation in Emacs:
gnus/rfc2104.el.  Unlike hmac-*, it doesn't use a macro to make a
specific function for each hash, but just takes the hash function as an
argument.  I'll try to see if that makes any significant speed
difference, and if one implementation could be reasonably deprecated in
favour of the other.

(Incidentally, all but one usage of rfc2104-hash are in places where the
SASL library could be used instead: nnimap.el, smtpmail.el, imap.el.)

Regards,
Magnus