using (in-readtable) to select a readtable

[73budden .]

Hi!

 I thought a bit about this.

  *readtable-alist* is just a kludge. First of all, it violates CL
standard: in-package changes readtable implicitly.

  The main problem of the approach is non-portability. It will work OK
while you're in SLIME, but it won't work in just the same lisp REPL
without SLIME.

  It would also work in a bit unexpected manner when you bind
*package* and then call read.

  It won't work in other CL environments. So I think *readtable-alist*
is harmful.

  I suggested verbose, but correct approach to deal with multiple
readtables vs source files. If my suggestion is accepted, it looks
like *readtable-alist* must be removed from SWANK.

  Evidently it is a breaking change and it can cause a trouble for
SLIME users, but
in the long run accepting my approach would be profitable.