add runtime value data type

lib/runtime/value.ml

@@ -0,0 +1,19 @@
+type t =
+  | Nil
+  | True
+  | False
+  | Int of int64
+(* | Obj of vtable * slots *)
+
+let equal v1 v2 =
+  match v1, v2 with
+  | Int x, Int y -> Int64.equal x y
+  | _, _ -> v1 == v2
+
+let to_string = function
+  | Nil -> "nil"
+  | True -> "true"
+  | False -> "false"
+  | Int n -> Int64.to_string n
+
+let pp ppf v = Format.pp_print_string ppf (to_string v)

lib/spice.ml

@@ -1,6 +1,7 @@
 module Syn = Spice_syntax.Ast
 module Lir = Spice_lower.Lir
 module Code = Spice_runtime.Code
+module Value = Spice_runtime.Value
 
 exception Error of string