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/eval.sx

@@ -430,6 +430,11 @@
               (else (error (str "ocaml-eval: unbound variable " name))))))
         ((= tag "neg") (- 0 (ocaml-eval (nth ast 1) env)))
         ((= tag "not") (not (ocaml-eval (nth ast 1) env)))
+        ((= tag "assert")
+          (let ((v (ocaml-eval (nth ast 1) env)))
+            (cond
+              ((= v false) (error "Assert_failure"))
+              (else nil))))
         ((= tag "deref")
           (let ((cell (ocaml-eval (nth ast 1) env)))
             (nth cell 0)))