[named-readtables] in-readtable does not touch swank:*readtable-alist* anymore (#8)

[73budden .]

> Skimming through that slime-devel thread, it's not clear there is a consensus on whether this is the right approach.

There is consensus about two things:
1. Understanding readtables in SLIME is useful and desired
2. Frobbing made by (in-readtable) is kludgy and potentially harmful.

But I'm not sure people understood that the change I suggest is breaking.

Currently, once readtable is associated to a package, every file with
that package implies that readtable due to record in
*readtable-alist*. So it is enough to issue (in-readtable) once in the
package and it sticks to that package until changed.

After suggested modification to SLIME and NAMED-READATBLES, explicit
in-readtable form is required at every file, and also every time we
issue in-package form in REPL.
We are also free to add records to *readtable-alist* manually.

How will user adapt to suggested modifications? Simplest way is to
fill *readtable-alist* in his initialization file.