Printing and reading special objects

[Stefan Monnier]

I think we should introduce two new variables:

- print-readably-function: a variable holding the function to call to
  print those objects which aren't usually printable in a way that's
  `read`able (e.g. those objects normally printed as #<...>).
- read-special-function: function called when encountering an unknown
  # thingy.

The second makes it possible to add a kind of reader-macro feature, but
the immediate intent would be to use those two variables via let-binding
rather than via globally setting them.  Together they would make it possible
to take a data-structure holding things like markers, obarrays, and
windows, print it via `princ` and then read it back via `read` (and
providing ad-hoc ways to print&read those special objects).  I think the
`print-readably-function` should be fairly easy.  I haven't looked at
the potential implementation difficulty of `read-special-function`.

Any taker?


        Stefan


PS: The print-readably-function would also be useful without
    read-special-function in a few cases.  E.g. it could be used in
    savehist to burp when part of the history contains unreadable data
    such as markers.  And it could be used in the byte-compiler to
    slightly simply the printing of the bytecode where we currently do
    some of it by hand in order to be able to write the "#$" special
    thingy since there's no object that prints as "#$".