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/runtime.sx

@@ -455,6 +455,10 @@
      let printf fmt = sprintf fmt
    end ;;
 
+   module Lazy = struct
+     let force lz = _lazy_force lz
+   end ;;
+
    module Stack = struct
      let create () = ref []
      let push x s = s := x :: !s