ocaml: phase 4 'assert EXPR' (+3 tests, 487 total)

Tokenizer already classified 'assert' as a keyword; this commit wires
it through:
  parser  : parse-prefix dispatches like 'not' — advance, recur, wrap
            as (:assert EXPR).
  eval    : evaluate operand; nil on truthy, host-error 'Assert_failure'
            on false. Caught cleanly by existing try/with.

  assert true; 42                          = 42
  let x = 5 in assert (x = 5); x + 1       = 6
  try (assert false; 0) with _ -> 99       = 99

lib/ocaml/parser.sx

@@ -622,6 +622,8 @@
                 (begin (advance-tok!) (list :deref (parse-prefix))))
               ((at-kw? "not")
                 (begin (advance-tok!) (list :not (parse-prefix))))
+              ((at-kw? "assert")
+                (begin (advance-tok!) (list :assert (parse-prefix))))
               (else (parse-app)))))
         (set!
           parse-binop-rhs