New `print' REPL option.

[Daniel Hartwig]

> ** New `print' REPL option.
>
> See "REPL Commands" in the manual for information on the new
> user-customizable REPL printer.

Since adding this option I have noticed that reader is also
customizable, using the undocumented variable ‘repl-reader’.  The
interface here is different.  A custom printer is wrapped in a
standard helper which, although convenient for the presumed common
case, inhibits printing of e.g. unspecified values and other more
elaborate behaviour.

Is everyone satisfied with the chosen implementation of this print
option, before we commit it to public API?  In particular, I wonder if
reader/printer setting should be made more symmetric by changing or
extending either.

Regards