examples: add all_any.pf (list_all / list_any correctness)

[jsiek]

What

Adds a new introductory-functional-programming worked example to examples/:
the classic higher-order predicates all / any over lists.

Because all and some are reserved keywords in Deduce, the functions are
named list_all and list_any. The file defines both by structural recursion,
includes a handful of assert sanity checks (using an is_even predicate), and
proves three correctness properties:

1. list_all_append — list_all distributes over ++ as a conjunction:
   list_all(xs ++ ys, p) = (list_all(xs, p) and list_all(ys, p))
2. list_any_append — list_any distributes over ++ as a disjunction:
   list_any(xs ++ ys, p) = (list_any(xs, p) or list_any(ys, p))
3. list_all_any_de_morgan — the De Morgan duality between them:
   (not list_all(xs, p)) = list_any(xs, fun z { not p(z) })

All proofs go by induction on the list and a switch on p(x). This fills a gap
in the existing examples/ set, which has sum, count, maximum, partition,
take_while, etc., but no Boolean-quantifier example.

Testing

• python deduce.py examples/all_any.pf → valid
• make tests-examples → all example files valid under both the
  recursive-descent and LALR parsers.

Notes

While writing this I hit one genuine beginner trap and filed two issues for it:

• Goal printer renders = inside and/or without disambiguating parens (hides a common beginner mistake) #786 — = binds tighter than and/or; the goal printer shows it ambiguously.
• Docs: Reference has no operator-precedence table (e.g. = vs and/or) #787 — the Reference has no operator-precedence table.

Claude session