Hello.

The attached patch adds support for the SCRAM-SHA-256 SASL mechanism.

I believe I still have commit access to the repository, so please review
it and I will attempt to commit the patch.  It was some time since I
last contributed anything so please be nitty so I will learn something.

Implementation comment: I wanted to put the code in the existing
sasl-scram-rfc.el file, but due to how `sasl-find-mechanism' works, it
isn't possible to have more than one SASL mechanism defined per
non-autoloaded file.  I didn't want to change the logic here, so I added
sasl-scram-sha256.el as a separate file instead.  If someone can figure
out a nicer way, that would be nice -- and might also fix the existing
hack at the end of sasl-scram-rfc.el that appear to be added due the
same reason.

Thanks,
/Simon