ocaml: phase 4 'lazy EXPR' + Lazy.force (+2 tests, 496 total)

Tokenizer already had 'lazy' as a keyword. This commit wires it through:

  parser  : parse-prefix emits (:lazy EXPR), like the existing 'assert'
            handler.
  eval    : creates a one-element cell with state ('Thunk' expr env).
  host    : _lazy_force flips the cell to ('Forced' v) on first call
            and returns the cached value thereafter.
  runtime : module Lazy = struct let force lz = _lazy_force lz end.

Memoisation confirmed by tracking a side-effect counter through two
forces of the same lazy:

  let counter = ref 0 in
  let lz = lazy (counter := !counter + 1; 42) in
  let a = Lazy.force lz in
  let b = Lazy.force lz in
  (a + b) * 100 + !counter        = 8401   (= 84*100 + 1)

lib/ocaml/eval.sx

@@ -116,6 +116,20 @@
         (fn (t) (fn (k) (has-key? (nth t 0) (str k)))))
       (list "_hashtbl_length"
         (fn (t) (len (keys (nth t 0)))))
+      ;; _lazy_force: evaluate the thunk on first force, cache result.
+      ;; cell: one-elt list whose value is ("Thunk" expr env) or
+      ;; ("Forced" v).
+      (list "_lazy_force"
+        (fn (cell)
+          (let ((state (nth cell 0)))
+            (cond
+              ((= (first state) "Forced") (nth state 1))
+              (else
+                (let ((expr (nth state 1)) (env (nth state 2)))
+                  (let ((v (ocaml-eval expr env)))
+                    (begin
+                      (set-nth! cell 0 (list "Forced" v))
+                      v))))))))
       ;; _hashtbl_to_list: returns [(k, v); ...] as a list of pairs.
       ;; Keys are returned as the stringified form used internally.
       (list "_hashtbl_to_list"
@@ -451,6 +465,11 @@
             (cond
               ((= v false) (error "Assert_failure"))
               (else nil))))
+        ((= tag "lazy")
+          ;; (:lazy EXPR) — create a one-element cell containing
+          ;; ("Thunk" EXPR env); _lazy_force evaluates and caches.
+          (let ((expr (nth ast 1)))
+            (list (list "Thunk" expr env))))
         ((= tag "deref")
           (let ((cell (ocaml-eval (nth ast 1) env)))
             (nth cell 0)))