using (in-readtable) to select a readtable

[73budden .]

Hi!
> Ah, good point. Budden, your current version completely sidesteps this
> mechanism. It should be easy to add it as a fallback though.
 I looked to a code once again and did some testing. I used emacs code:

(defun show-guessed-readtable ()
  (interactive)
  (message (slime-eval '(cl:string
(editor-hints.named-readtables::readtable-name
swank::*buffer-readtable*)))))

and now I'm almost sure that my code uses *readtable-alist* if it does
not find (in-readtable) statement in a file. So fallback works
already.

I also found a problem: I can't change readtable through
(in-readtable) in a slime repl buffer. I guess *readtable* somehow
gets bound around real evaluation, but I dont know how to avoid it.
Currently I have no idea what to do. It looks like some special
actions must be done for repl buffers as we must take *readtable*

So I created a pull request https://github.com/slime/slime/pull/259
(hope I did it right). But code needs fixing.