bug#25295: Represent eieio objects using object-print in backtraces and edebug

[Stefan Monnier]

> Actually, print.c is already setup to be (at least partially) reentrant
> (for the case `printcharfun` is a function), so maybe making it fully
> reentrant won't be too hard.  But we'd still want to provide some ad-hoc
> prin1-like function which continues the print job rather than starting
> a new nested one; this is needed e.g. if the custom printer wants to
> print a value V but V has already been printed so it should be printed
> as a reference #NN# to the previous one.

Looking further at the code, maybe this isn't even needed, since the
print-number-table can survive calls to print.  But that can get ugly
when your printer code also want to call `print` for "unrelated"
purposes (e.g. to print debug messages in a side buffer).
And I'll stop thinking about interaction with concurrency.


        Stefan