wanted: SWANK:DECLAIM-INDENTATION

[joaotavora]

bug imported from https://bugs.launchpad.net/slime/+bug/627310

If I remember correctly, here's something along that line hidden in the
slime-indentation contrib already. I think it should go into swank proper
because swank.lisp already has most of the code for it, and at least I
want to use such a facility without being forced to use the emacs-side
of slime-indentation.

It will also make it a little bit more convenient for people to depend
on it. (They just have to check for presence of SWANK.)