ocaml: phase 6 Hashtbl (+6 tests, 332 total)

Backing store is a one-element list cell holding a SX dict; keys
coerced to strings via str so int/string keys work uniformly. API:
create, add, replace, find, find_opt, mem, length.

_hashtbl_create / _hashtbl_add / _hashtbl_replace / _hashtbl_find_opt /
_hashtbl_mem / _hashtbl_length primitives wired in eval.sx; OCaml-side
Hashtbl module wraps them in lib/ocaml/runtime.sx.

lib/ocaml/runtime.sx

@@ -266,6 +266,19 @@
    module Printf = struct
      let sprintf fmt = fmt
      let printf fmt = print_string fmt
+   end ;;
+
+   module Hashtbl = struct
+     let create n = _hashtbl_create n
+     let add t k v = _hashtbl_add t k v
+     let replace t k v = _hashtbl_replace t k v
+     let find_opt t k = _hashtbl_find_opt t k
+     let find t k =
+       match _hashtbl_find_opt t k with
+       | None -> failwith \"Hashtbl.find: not found\"
+       | Some v -> v
+     let mem t k = _hashtbl_mem t k
+     let length t = _hashtbl_length t
    end")
 
 (define ocaml-stdlib-loaded false)