added loxstd with clock and exit

lib/dune

@@ -1,5 +1,6 @@
 (library
  (name Lox)
+ (libraries unix)
  (preprocess
   (pps ppx_deriving.show)))
 

lib/loxstd.ml

@@ -0,0 +1,28 @@
+open Environment
+open Value
+
+let clock : native_function =
+  let fn = function
+    | [] ->
+        let time = Unix.gettimeofday () in
+        Number time |> Result.ok
+    | _ -> assert false
+  in
+  { name = "clock"; arity = 0; fn }
+
+let exit : native_function =
+  let fn = function
+    | [ n ] -> (
+        match n with
+        | Number n when Float.is_integer n -> Int.of_float n |> exit
+        | _ ->
+            Printf.sprintf "Must call exit with integer, received %s instead" (show_lox_value n)
+            |> Result.error)
+    | _ -> assert false
+  in
+  { name = "exit"; arity = 1; fn }
+
+let init_std (env : environment) =
+  let register_fn fn = Env.define_global env fn.name (Function (NativeFunction fn)) in
+  let _ = List.map register_fn [ clock; exit ] in
+  ()