using (in-readtable) to select a readtable

[73budden .]

Hi!

> it certainly can, but does it?
Just see the source:

https://github.com/melisgl/named-readtables/blob/master/src/named-readtables.lisp
in-readtable calls %frob-swank-readtable-alist .

> and if it does, then why?
I don't know, but I think it was done for some purpose and something
may depend on it.

> let's change it so that it doesn't alter *readtable-alist*!
This seem to be ideal solution at a glance, but I think we should find
the reason why it was done this way initially.