bug#55481: mostly nonrecursive printing [PATCH]

[Mattias Engdegård]

Printing deeply nested values typically crashes Emacs by running out of C stack 
since the printer uses C recursion to traverse nested data structures. This is 
a long-standing problem that keeps turning up; see bug#52753 for one example.

The attached patch eliminates most of the recursion: conses, vectors, records, 
hash tables and char tables are now traversed without consuming C stack. This 
should cover the vast majority of objects that form deep structures.

Of note:

- Performance seems to be slightly improved (about 2.5 % faster for printing a 
long list of symbols) but that's not really the point of the patch.

- The patch does not attempt to fix the bogus #N notation for circular lists 
(bug#55395) but tries to stay bug-compatible for easier comparison.

- Some special syntax is context-conditional: (\, X) is only printed as ,X if 
surrounded by a positive number of backquote forms. It's not clear what we gain 
from this; using the special syntax for the backquote, comma and comma-at forms 
unconditionally would simplify matter without any apparent inconvenience to the 
user. Right now, the patch does not remove recursion for printing these forms.

- This patch does not address reading nested values, where a similar problem 
exists.

print-nonrec.diff
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