entry separator for printing hashtables

[Ted Zlatanov]

Since hashtables can get pretty big, it would be nice to have a
separator between entries when they are printed.  Could that be provided
at the top level, e.g.

(let ((hash-table-print-separator "\\\n"))
   (format "%S" my-hash-table))

or in some other way?  It would make large hashtable dumps like the ones
from the new registry.el and gnus-registry.el much easier to read and
debug.  In the old gnus-registry.el I had special code to break up
alists like that but it was slow so I'd rather ask Emacs to do it for
me at the C level.

Thanks
Ted